For most traditional artwork, outside, natural light photos without a flash usually are the best photos. However, indoor photos can work too! The best lighting is indirect such as on cloudy days, early mornings, or late evenings. High-resolution photos are the best. At times, I am willing to approve "poor" photos with the client's acceptance that the artwork might differ from the actual, real-life subject. Also, don't worry about backgrounds or unwanted objects - I can easily edit, delete, and add objects as desired!
For non-traditional, or expressive artwork, more liberty can be taken with reference photo quality.
Examples of Bad Reference Photos
Hover over photos to see why these aren't the best reference photos.
Examples of Good Reference Photos
Hover over to see why these are better reference photos.