Writing the Art History Paper has very useful information for writing about art. In fact, some of it is so good, I feel the need to quote it here.
Be sure to analyze as you describe. The comment we’ve heard most often from Art History professors is that students often describe a work without considering the argument they ultimately intend to make. Consider carefully the purpose of your paper, then choose and organize your descriptive details so that they illustrate not only the painting, but your analysis of it.
Be aware that paragraphs in Art History papers are often constructed so that the topic sentences are at the end. Typically, students are trained in their composition courses to begin their paragraphs with a topic sentence. However, in Art History, students will often find paragraphs constructed with the topic sentences at the end. In these kinds of paragraphs, details build towards an observation or argument. When creating a paragraph that ends with it a topic sentence, you need to be especially careful that your details are well-chosen and logically expressed, and that they build towards the point you are making.
Think about the ways in which you want to structure your papers. Structure them chronologically if you are discussing an artistic movement or a specific artist’s progression; spatially if you are discussing the elements of a specific work; relationally if you are discussing a work in relation to a movement or another work; and so on.
Avoid the subjective “I.” You want your reader to feel that your point of view about a particular work comes from some formal aspect of that work, and not from some very personal response of your own. This is not to say that your personal response to a work is irrelevant; rather, it is your job as a critic or scholar to figure out what formal aspects of the work created your response, and then to explain fully how and why.
This is from Dartmouth, a very good school.