I've used a local photography studio for my large scale paintings- particularly for work that was going to my publishers...
But, for regular pics, I lay the painting on the floor, and stand above it. It stops there being any angle issues, and keeps the light even.
I try to take my photos in the morning, on a sunny day- so there's plenty of natural light, but no glare.
Obviously I don't use a flash, and I edit the photos to tone down, or up, the temperature & exposure.
I don't use a dSLR- I can't figure them out... but, I use my macro settings on smaller work, and adjust the other settings according to the situation.