- Prop your painting against a wall on a sunny day.
- Put the camera on a tripod/box aimed straight onto the painting to minimise distortion.
- Minimise “bulging” by zooming in and physically moving the camera back until the painting fits the frame.
- Take several photos, some with the painting in the shade
Source (and more info) at EmptyEasel: emptyeasel.com/how-to-photograph-your-artwork-for-a-portfolio-or-the-internet