Even though this sounds like a homework question, I take a stab.
To me, as far as representational pictures (as opposed to abstract), one big difference between a photo and a painting is that a photo is taken with one "eye" and a painting is made with two (usually). What does this mean? Well, one eye cannot perceive depth or three dimensions the way two eyes can. Therefore, a photo inevitably has a flatness despite there being clues to it representing a three dimensional world.