In order to get the best image, you may find it easiest to lean most images against an exterior wall. If you in an environment like our rainy Seattle - be careful; keep the paintings safely tucked under an overhang or otherwise protected from the elements.
You may say, Why not leave them on the wall? The reason is that the light inside is usually much worse than indirect light outdoors. In a clean, well-lit place, though, you can leave your artwork on the wall and shoot it where it hangs. More power to you.
If your pieces are small (less than 10" x 14"), then you may be able to lay them flat on the ground and shoot from above. Larger work will be more easily shot if you lean it up against the wall. Either way, you may also find it beneficial to place a large piece of white cardboard or foam core behind your painting. This will eliminate any distracting background and allow the viewer to focus on your artwork. You will find this helpful especially if you are shooting slides, since your cropping options will be limited.
Both of the following images were shot leaning up against the wall; one with flash and one without.
 Leaning Against the Wall; Flashed |
 Leaning Against the Wall; Natural Light |
It was much easier to shoot the photo (and subsequently crop the picture in Photoshop) by leaning it against the exterior wall of our house.
Whether you lay the art flat or lean it up against the wall, your challenge will be to shoot the artwork straight on, without skewing the frame or edges by leaning one way or the other. Imagine the plane your film is occupying in space and try to keep it completely parallel to your painting. Your goal is to hold the camera as if you were to lay the film flat against the painting. Use a tripod if you have a hard time keeping the film plane parallel or keeping the camera steady.
 Askew and Flashed |
 Straight and Cropped |
Also notice the glare in the flashed photo.
If your painting is nicely framed, include the frame in your image. You do not want to cut off the edges of a good frame any more than you would want to crop the edges of your painting. Unframed artwork can be shot with just a bit of space around the edges.
1) How to Photograph Paintings - Introduction
2) Turn Off Your Flash
3) Up Against the Wall
4) Dealing with Frames
5) Dealing with Glass
6) Digital Fixing