I have no idea why guys send dick pics generally, least of all why a client would send a dick pic to a provider. Interesting theories on general dick pics rom the linked article in Psychology Today...
"There really aren't any research studies looking at this question, and so all we can do right now is speculate, though with some informed psychological wisdom:
It's most likely that this behavior represents an aspect of men's misperception of female sexual interest. Men love the idea of receiving such pictures from strangers, and they assume women do too. Men notoriously misperceive women's sexual interest in them and project their own sexual interests and desires onto women. In this situation, men really are hoping and thinking that you'll be turned on and send them a pic in response.
It's probable that some of this connects to the fact that in an anonymous environment, people, and especially men, are likely to engage in more sexualized behaviors. Psychological research has demonstrated that in anonymous environments, people, both men and women, engage in fairly casual sexual behaviors, including exhibitionism.
Male mating strategies have always included an element of 'boldness,' where men who are bold and brash sometimes garner female attention they wouldn't otherwise receive if they were nice and polite. This is a key tenet behind the Pick-Up Artist strategies, where men are encouraged to be bold and impulsive. So, the 'shock value' is a way for men to get attention. And negative attention is better than no attention at all.
It's probable that at least some of these men receive a sexual thrill at the idea of an unknown woman seeing their genitalia. It may be an aspect of exhibitionism, and some of these men probably masturbate as a part of the act, imagining that woman seeing the picture they sent. The fact that a woman rejects them for it is not salient, because for many such men, it is the woman's disgust and rejection which is actually part of the turn-on. These are likely the same men that used to be in trenchcoats on street corners. In Ohio, one man was dubbed 'The Naked Photographer' because he would jump out and expose his penis to women, and take a photo of their reaction. When he was caught, he later admitted that he would masturbate to the pictures of the women’s reactions. Famous French philosopher Jean Jacques Rousseau used to hang his naked buttocks out of alleyways, hoping that some women passersby would spank him for being such a naughty boy.
Men fear sexual rejection, and by sending pics of their genitalia, they are almost getting 'pre-approval.' This way, they get the chance of rejection out of the way early, so they don't have to worry about being rejected or shamed once they drop their pants on a real date.
