(n) an urgent need to defecate, usually associated with diarrhea, in which the bowels begin to move before the victim has reached the shitter
As Bob raced down the hallway, he clenched his butt-cheeks together as tightly as he could to stave the impending explosion of fecal matter. This was definitely going to be a photo finish.
When you have an extremely urgent bodily need to jetison waste, and you are in the process of literally running to an establishment to get rid of said waste and you are not sure that you are going to make it before...you know...
Oh dammit i really have to shit, and the nearest toilet is 3 blocks away!!! oh man this is gonna be a photo finish!
As you're about to climax while doing someone from behind, you grab the nearest family photo and ejaculate onto it.
As Ted was banging Patty from behind, he knew it was time to close the deal. He grabbed the nearest framed photo and exploded onto it. The next morning, Ted saw that he had Photo Finished onto a picture of his own sons, Brad and Paul.