Even lighting across the face of the painting is recommended.
Daylight works very well (outside on an overcast day or in the shade on a sunny day).
If shooting indoors, you want to use daylight bulbs, not soft white. Soft white bulbs will make your photos look yellowish.
Brighter light is better. If the light is not bright enough, the photo may be out of focus or grainy.
If the paint is glossy, an indoor lighting system with polarized film is a good solution for removing glare. This is more labor-intensive but relatively easy once you set it up.
If you want to emphasize the thickness of the paint, place the lights above the painting pointing down. This will mimic the lighting in which most artwork is viewed, and when you print the image, the viewer will see the shadows created by the texture as if they are really there.
To watch a video illustrating the DIY four-point lighting system and polarized film, click here.
