I had two main "theory" goals in writing this set: A) providing a sense of "art history" and B) filling in gaps in the canon.
A: Art History
Starting with art history, I think one of the things that visual arts writing in quizbowl tends to overlook is contextualizing works and artists to their place in the real world. This isn't always overlooked, but I've gotten the feeling that this isn't a primary goal in the minds of visual arts question writers--instead, it occasionally makes it into questions because they are good, unique clues.
Our first instinct when writing questions is to interpret a work or artist in a vacuum. This is typically a good isntinct--most clues should focus primarily on the work or artist. But I think we can use more questions like Laocoon or clues like the one about how Nefertiti's bust was given a body for a Venice Biennale exhibition. Without a healthy dose of these kinds of clues, it's easy to lose sight of how a work did or can function in a larger context. Now, I realize that I'm still being a little vague on what sorts of questions and clues give rise to the "context" I'm looking for. So how does one choose these kinds of clues? I don't have a specific process, so instead I'll give examples:
1. Biographical clues that are relevant or evident in the actual art, with the connection explained in the question. For example, the Finals tossup on USA led in with several lines on George Inness:
A lot of the time writers will stop at "Swedenborg" or "divinity of landscapes," without talking about how these influences on the artist manifested themselves in the art. Even though "bucolic scenes with cloudy skies" isn't the most evocative clue, it grounds the players in something they can visualize, so that they can make visual sense of the philosophy. Here's another example:While in Rome, an artist from this country was introduced to the philosophy of Swedenborg, which led him to believe in the divinity of landscapes, in which he typically paired bucolic scenes with cloudy skies.
This describes The Swimming Pool, one of Matisse's cutouts. That quote hardly provides any decent visual description of the work itself. And is the fact that Matisse "adored the sea" really all that helpful in figuring out that it's The Swimming Pool that we're talking about? I don't think so. The reason this quote is important is because Matisse had to resort to cutouts after he was confined to a wheelchair. Most of the rest of the tossup focuses on the visual elements of Matisse's cutouts, but this clue is there to reward knowledge of the "story behind" the cutouts. I think it's important to understand where works come from, rather than only formally analyzing their visual elements. So I tried to give people extra points for digging deeper. The best buzz I saw all day was Jacob Reed getting 20 points for knowing this quote.Regarding the largest of these works, its artist remarked, “I have always adored the sea, and now that I can no longer go for a swim, I have surrounded myself with it.” That nine-panel mural was originally located in the dining room of the Hotel Regina in Nice and shows a starfish on either end of a frieze filled with acrobats and bathers.
2. Artists referencing other artists. This is an important one. It points toward how artists have influenced others that came after them, and this can be important in two ways.
One way is as a commentary on the original artist or work in the context of its original creation or display. For example, the question on Snow-Storm: Hannibal Crossing the Alps included a clue about how the painting is thought to be a response to David's Napoleon Crossing the Alps, and the different portrayals of the titular actions speaks volumes about an English painter and a French painter's views on the invasion of Italy.
The second way is as a commentary on the original artist or work in the context of art in the context of the reference. Little Warsaw's Body of Nefertiti is a good example. I suppose the rationale for that work was something like this: "The bust is always viewed as a bust, but how can we try to unlock more about the figure embodied in the work? Why, let's complete the rest of the body!" Whether the body is accurate or sheds any light on Nefertiti herself isn't all that important, as far as the question goes. Instead, my goal was to point out that artists today are still capable of reinventing works that have been studied and over-analyzed by scholars for centuries. That's an important part of how art develops, especially in our age, and I thought it would be helpful to incorporate some of that into visual art questions.
Another example is Woody Allen's incorporation of the Hall of Mirrors scene from The Lady from Shanghai into Manhattan Murder Mystery. To me, the radically different ways in which the same dialogue was used in those two films was striking--Welles used the lines as the climax to a weird femme-fatale thriller, while Allen used them as an abrupt and gratuitous film reference in order to maintain the lighthearted adventurousness that characterizes the rest of the film. These references are not only interesting in themselves, but are good pointers toward how the role of certain works can change over time.
3. Artists referencing important aspects of the times in which they lived. This is something question writers do pretty well already, I think. Examples of this in my set are the tossups on Byzantine emperors in Renaissance painting, Napoleon, the KKK, and Sacco and Vanzetti, plus clues like the one describing Puvis de Chavannes reactions to the Franco-Prussian War.
4. Straight-up "art history" questions. There weren't a ton of these, but examples include the tossups on Napoleon's art program, Peggy Guggenheim, Jackson Pollock, This Is Tomorrow, Altmann v. Austria, Naples, the Haussmannization of Paris, and Giverny. Putting too many of these questions in a set risks altering the category of "visual art" completely, so I don't recommend overdoing it.
4a. Clues about shows and exhibitions. This kind of comes under the pure "art history" label. But galleries and organizations played important roles in both the development of artists and the selection of who gets to be famous and who doesn't. My favorite exhibition clue was this one:
B: Gaps in the CanonAn artist from this movement showed a bound and bloodied Christ being swarmed by a mob of angry women, who reach toward him with their arms. That artist of Christ aux Outrages was expelled from this movement after refusing to exhibit his paintings alongside van Gogh’s Sunflowers and trying to duel Toulouse-Lautrec. Anna Boch, the only female member of this movement, bought The Red Vineyard, the only painting that van Gogh ever sold. A Symbolist in this group showed a shepherd with his face pressed up against that of a sphinx with the body of a cheetah in The Caress. This movement was founded after a genre scene of a woman eating (*) oysters at a breakfast table was rejected by both the L’Essor and Antwerp Salons. Organized by the art critic Octave Maus, this group included Fernand Khnopff and a man who placed a banner reading “Vive la sociale” in one work. For 10 points, name this group composed of a certain number of Belgian artists, including James Ensor.
ANSWER: Les XX [accept Les Vingt or The Twenty] 
There's no way I met this goal perfectly or comprehensively, but whenever I thought, "Hey why isn't X tossed up more often?" I tried to incorporate it into this set. I suppose this sort-of comes with just trying to be creative with question writing, so it's not visual-arts-specific. Anyway, here are two general categories in which I tried to make the canon more complete, without really expanding it:
1. General knowledge, or knowledge from other academic areas. Questions and clues relevant to this point include the tossups on the Alexander Mosaic, Hearst Castle, Saint-Chapelle, soccer stadiums (using architectural clues about buildings you'd normally see from watching soccer on TV), and war memorials. Clues in this regard include the one about Roger II's mantle from the Sicily tossup, and the one about the Catalan Atlas in the Mali tossup. Other things that might have been "general knowledge-y" include Maxfield Parrish and George W. Bush (I will never be sorry about that one). Film was an area where this principle was most dominant. I tried to pick a lot of topics from popular films that are watched for their entertainment value and not necessarily their artistic value, and then tried to reward people for paying attention to the artistic elements.
2. Famous things that don't really come up as independent tossups. This is pretty self-explanatory, but I encourage people to think about this for higher-difficulty tournaments. Examples: Parson Weems' Fable, the Paris Opera ceiling, The Steerage, the JFK Presidential Library, and the Johnson Wax Headquarters.
I'll close with a little bit about my process. First, I tried to be very methodical about my answer selection: When I started this project, I made a spreadsheet with a certain number of cells reserved for each medium, a certain number of cells reserved for specific chunks of time within each medium, and a certain number of cells reserved for specific countries or styles within each time period. For example, there were 8 tossups reserved for painting from 1870-1900. I made sure that there was at least one Impressionism question, Post-Impressionism question, American question, non-USA/UK/France question, etc. Setting up these niches meant that even if I decided that an answer was unworkable, I knew exactly where to look in order to find an alternative, without messing up the overall distributional aesthetic of the set.
Second, I had over a year to write this set. Of course, since this is quizbowl, two-thirds of the set was finished in the six weeks leading up to the tournament. But having a year still helped immensely, and I actually think it was the reason why this tournament was successful. Basically, having a lot of time means you can reflect on your answer selections. Answers were added periodically over a long period of time, meaning I could really think about whether I wanted a topic to take up space in my tournament.
Third, I relied on art history courses I've taken and museums that I've been to in order to ground my overall sense of what things are important and what things aren't. For a tournament at this difficulty, I think this was important because I was putting in clues about artists and works that have never been asked before in quizbowl.
Fourth, for all of the context clues, my go-to source was either Wikipedia or the website of the museum in which the work is held. If I was using Wikipedia, I made sure to cross-reference all of the clues I took from there to make sure 1) that it was true and 2) that it was actually important. For example, I found the clue about how a copy of Maxfield Parrish's Daybreak was in a quarter of American homes at one point on Wikipedia. But I looked at newspaper articles, art history blogs and websites, etc. Now, you might be thinking--how academic or reliable are those sources? Maybe the answer is "not very," but the idea was that if something appeared enough times in different, independent sources, then it was probably both true and important.
Fifth, for most of the direct quotes from artists or commentators, I relied on art books (usually catalog-style), generally available on Google.
So there you have it. This is what I probably did sometimes, and if you think it's both correct and helpful, then I'm glad to have shared my method of writing with you. If not, I apologize for my long-windedness. And thanks to all for your enjoyment of and kind words about this set.