A monograph is an in-depth study of a particular artist. This form is suitable for a thesis, as it is broad enough to be significant, but limited enough that it won't get out of control. Generally, a simple biographical account will not fulfill the requirements of a thesis; you will need to approach the work and life of the artist with the intention of proving a point or making a significant connection that has not been made in other research. As an example, if you are interested in the work of the French painter André Masson, you could write about his experiences as a soldier in World War I, and extrapolate from that to explore the impact of war and combat on the imagery of Masson and his contemporaries. This gives specific information, but contextualizes it within a broader and very significant historical setting.
If you are more inclined toward theory than history, that is a perfectly legitimate basis for a thesis. A theoretically based thesis roots itself in a particular way of looking at art or understanding art, rather than a particular artist or historical period, although it will almost certainly address artists and histories. For example, a thesis that was based on a postmodernist theoretical perspective might explore the work of the 1960s or the Pop Art movement, but rather than focusing on Andy Warhol, it would use Pop Art and Warhol as examples and as vehicles to support a certain point of view, that the meaning of the art object is dependent on the viewer, or that all ideas in art are ultimately recycled from other ideas.
Synthesis is a challenging form of theory, and one that can quickly go awry if you aren't careful. In order either to synthesize or to rebut other theories, you must have read them exhaustively and have an extremely thorough understanding of their details. You could make a very impressive theory by, for example, exploring Walter Benjamin's theory of the "aura" of the art object within the context of Gramsci's theory of hegemony, but if you don't really know what you are talking about, your committee is sure to notice, and they won't be impressed. Take on the most challenging subject matter that you believe you can handle, in order to make an impressive thesis and to learn as much as possible, but don't overextend yourself. Know your limits, and challenge those limits intelligently.