Welcome to Fluther.
I had a friend several years ago who told me this was a recurring thing for her and some others that she knew of. I couldn’t imagine that “unknown” guys were sending these photos to women that they also don’t know well. Really, sending any of these photos any time seems “off” to me, but especially when neither person even knows the other.
I recommended to her that she save some of the photos, and when she gets one unwanted photo from one strange guy, that she mail him back… two photos of other guys.
She told me that it worked like a charm. Some guys who had been routinely sending these photos (so I guess that she was not unknown to them, anyway) stopped cold. She still gets the odd photo from time to time, but she can live with that, I guess.