No.  They just need to be photos!  I’ve created pieces of art using images from Facebook and Instagram, as well as regular photos, and scans of very old photos indeed.  I digitally correct and enhance each image so it looks its very best; though if they weren’t of a reasonable starting quality to begin with (think blurry, thumbs over the lens, overexposed/underexposed) I’m very limited as to how much improvement I can make.