How can I prevent and correct perspective distortion when photographing framed artwork?

What you’re seeing is perspective distortion: the camera was tilted relative to a flat subject, so the rectangle appears trapezoidal.

To prevent it next time:

• Keep the camera sensor parallel to the artwork.
• Aim straight at the center of the piece.
• Raise/lower your shooting position instead of tilting the camera.
• A tilt-shift lens can also help, but it’s an expensive solution.

To fix it in Photoshop CS4:

1. Open the image.
2. Select the Crop tool.
3. Draw the crop over the whole image.
4. Check the “Perspective” option.
5. Drag the crop corners so they align with the corners/edges of the frame.
6. Press Enter/Return to apply.

Photoshop can correct this well for many images, though you may lose a little of the edges during the crop. If the angle is extreme, a reshoot may still give the best result.

Other software mentioned by users includes perspective-correction tools/plugins, but Photoshop CS4 should already handle the basic correction you need.