I get a free/fun app instead of $100. Instead of a .01% chance, I have a .00000001% of appearing in an objectionable ad (my photos just aren't as good as pros, and I'm just not that pretty). Add to that-- if this happens, I certainly won't be the first one it happens to and will almost certainly have an opportunity to delete my account when I start hearing about this happening in the wild. Even if I don't delete it, the chance of my crappy photos getting found/used out of the MANY BILLIONS on instagram also seems laughably small.
Statistically, this could bite me-- but I have to figure that the chance is so close to 0% that the (small) reward of using Instagram is worth the (trivial) risk. I also tend to dismiss concerns around lightning, sharks, and hijackers.