If you have ever tried, you know photographing a painting can be a challenge. While there is not much room for subjective interpretation by the photographer, there can be technical difficulties. The finished product wants to be perfectly square to itself, but often the canvas is not. Proper camera placement is a must to minimize distortion that may occur from even the best lenses. The work must be consistently lit across the entire face of the work. Add in glare in glass, or just the medium being used (i am talking to you oil paint), and there are a lot of variables to wrestle with. You should just have me photograph your painting.
One note of consideration. Some times frames and glass present obstacles that will impact the final image we capture. This is because of shadows that can be thrown from the frame, and a limited set of options for lighting. Additionally, glass itself is another barrier between your art and my lens, and in it's own way can distort your art, particularly texture. So, if your art is framed and behind glass, I can shoot it, but you may consider removing it for a better final product.