Eh, just to provide a different guy take, I would take it as a bit creepy if a woman sent me nudes before some sort of friendly relationship had been established. Not so much concerned about 'trashy' but more disturbing and raising red flags. Someone who would do this is not for me, even if just for a fling.

As far as OP's question, I think there are two types. I notice in a lot of these messages on this site the pics don't come out until a rejection has already occurred, I think these, type 1, are meaning to be obnoxious as sour grape revenge for rejecting them. There's also the ones that come before rejection, really before any meaningful communication, these I interpret as someone who might not have the balls to act on an urge to flash someone in real life but might do so by and electronic medium where the stakes are lower, I think they're getting a thrill out of exposing themselves, and may even occasionally get response to it, fueling the behavior. Just theories but that's my take.