Blame the Internet for this one.
I’m assuming these are guys you’ve met online, and taken to to the “phone” level. If that’s the case, what they’re looking for is proof that you’re not, in douchespeak, a SIF: Secret Internet Fatty.
I don’t make up the terms, folks.
Basically, they want to know you are who you say you are. Whether or not you reply in kind is completely up to you; they don’t have a right to pictures.