I regularly print coloring pages for my kids and the process is usually the same: 1) they name a character, 2) I search Google images, 3) we scroll through and pick 2-3 to print.

I asked them if they would want to use existing photos for coloring and they didn't express any interest. I think I know why -- people and places (i.e. most of the photos we have) are not as interesting to color as dragons, fairies, favorite show characters etc.