The answer I provided was multipart. For starters, a man who is not at least a little proud of his penis and who does not think his penis is worth an early episode of “show and tell” will not send such picture. So, in a
way, this is the way of men to show their “endowment”. Of course, these pictures are always of erect penises, never flaccid. Men want to “advertise” their merchandise and start a game of “peek-a-boo” that will be reciprocal. They want to receive a like picture in return.
But why, oh why? Well, men are visual creatures. Not that women do not get aroused visually, but men are notorious for being sexually stimulated by visual “aids”. And, although this argument may sound stereotypical, a man who sends a picture of his penis right after an exchange of phone numbers is
suggesting that what he is interested in is a sexual relationship ASAP; no “lovie-dovie” involved, just sex. The pictures of the proud possession of men, their erect and perfect penis, is their oscillating flag letting us women know that, if we trespass, it should be clear that we are not going in for love or romance, we are going in for sex. And men believe we will be as enticed by the picture of their merchandise as they would if we were the sender and they were the recipient.
I advise that, if the receiver does not appreciate the picture, express so. Just like when you go to a store and they offer you a piece of clothing or a pair of shoes, can kindly decline and say “beautiful, but I am not
interested”. If you are, just play along and be warned.