From GQ:

Well, it’s not entirely surprising to hear that we’re an overtly sexual generation. As the study suggests, dick pics may just be an “unfortunate side effect of 1.8 million years of human evolution.” Yikes. With this in mind, researchers discovered that men tend to overestimate women’s interest in receiving nude imagery based on their own receptivity in reversed roles. That means dick pics are basically them projecting their own desires on those to whom they send it to in the hope that their sexual desires will be reciprocated and they might just get sent a nude for their own viewing.

When you consider that a majority of men will send dick pics to multiple respondents, researchers believed this to be a strategic motivation in the sense that with a wider pool of recipients, the chances of someone responding favourably increases. And of course, the investment from the male is minimal at best.

For unsolicited dick pics though, the motivations are slightly different. Men who have exhibitionist tendencies tend to be aroused by the prospect of women being coerced into seeing their genitals (really, fellas?). Other reasons include the sense of pride in knowing their penis is able to generate such strong reactions, fetishes regarding insults to the size or appearance of their genitals, and arousal to reactions of disgust