Taking high quality photographs of a painting is difficult because of many artifacts that show up in the final result. Some of the artifacts you need to look out for are color balance, keystoning, twist, focus, exposure, glare, and reflections.

Color balance: Use natural light. Place your painting face up on the ground outside in the shade. If placed in direct sunlight you will get washed out areas of the painting. If shooting outside is not possible, put the painting on the floor of a sunny room. If you must use artificial light, try to avoid incandescent lights since they will add an orange tint to the image.

Keystoning: This makes the painting look like a trapezoid. It happens when you take the photo at an angle to the surface of the painting. Position your camera over the center of your painting and face it directly down toward the surface.

Twist: The sides of the painting should be parallel with the sides of the photograph. Orient the camera so that the sides of the camera are parallel with the sides of the painting.

Focus: Hold the shutter button down for a few seconds to allow the autofocus in the camera to set the focus properly.

Exposure: As with the focus, hold the shutter button down for a few seconds before letting it up to take the photo. That will give the camera time to set the exposure.

Glare: Turn off your flash. The flash will cause bright spots in the image.

Reflections: If your painting is behind glass, it is difficult to avoid reflections. If at all possible, remove the painting from the frame to get it from behind the glass. If you must shoot the photo through the glass, place yourself in a position that minimizes the amount of light falling on you. You can also try shooting the photo at an angle, placing the camera off to the side of the painting. That will cause keystoning, but it is easier to correct keystoning in image processing software than it is to remove reflections.

Good luck!