Shooting a painting so that it's a good representation of the actual piece is super hard. When I shoot my oil paintings, I do so in indirect natural light with no flash (i set them on a table near a window on a cloudy day, and shoot the image from above). When I shoot my gouache paintings I use the flash since the paint isn't shiny like oil or acrylic (I also do this in indirect sunlight). Once the images are uploaded, I edit the beejesus out of them in Photoshop, paying a lot of attention to brightness/contrast and color. Once they look good I save them at 72 ppi.
I think it's also helpful to take a bunch of images of your painting so that the customer can get a feel for what it might look like in different situations. I generally do one full sized image, a detail, and a shot of the the actual painting from an angle so that you can see the paper on which it was painted.
I hope that helps some!