Depends on what you're going to do with the picture. Most of the pictures on here probably haven't been taken/created por the user who uploaded it, so if you're going to copy the exact picture in your painting and you're going to sell it, then legally you'd have to ask permission from the original owner of the fotografia (though to be honest, I don't think anyone could prove that you had used the picture unless you'd copied it right to the last detail). If, however, you just wanted to paint a picture for fun and not for commercial purposes, then you can use reference from pretty much any picture really.
