Bottom line: do not take your pictures indoors. You won’t be able to capture the details of your painting to its full potential. Taking photos outside helps you see every detail in your painting. You don’t even need to use a fancy camera… I just use my iPhone!
With just your phone you can easily capture the small details in your painting. I try to take my photos around sundown so when the sunlight hits the painting, it adds a warm effect which overall helps the painting look better.
Don’t ever take a photo with flash, it tends to wash out the tones of the painting. Turn your painting at a 45 degree angle towards the sun to get the best lighting.
If for some weird reason you don’t want to take a photo of your painting outside, at least try to take a photo of the painting using the lighting coming from windows.