Aziz Ansari talked about this on The Jesselnik Offensive a few months ago. On his latest stand-up tour, he would conduct an informal poll along the lines of "Clap if you're a woman. Now clap if you've ever been sent a dick pick." And the results were always, in his estimation somewhere above the 2/3 mark or something. Way higher than one would expect.
Of course, Anthony then made it even funnier by adding, "Now clap if it was Aziz who sent you the dick pick."
Anyway, I've never sent one (seriously - nobody wants to see that), but I can only guess that guys send them because they want reciprocation. And any picture of a nude woman is about as desirable as a picture of a cock is undesirable.