What does it mean when a guy randomly sends you a picture?

He is slowly making it easier for him to send pics of his penis.

You talk every day and he apparently likes that. He likes you and senses that you like him. It does not matter if you have not told him that you like him if you make it obvious by how you act and talk with him. Whether it be talking, texting, or sending a picture, he is COMMUNICATING with you, and expressing that he likes you.

Let him know you enjoy his pictures and send him some sometimes.

No, there does not seem to be any underlying interest in sexual pictures. It seems to be clean enjoyment of friendship.

He's making the lamest play ever to have you send him a sext of your hoo-hoo?
I dunno.

He thinks you like him, and he likes you, though, to be doing that. Reply with something like "wow. hot." and see if that gets him to use his words.

I think he is just showing his affection. It just sounds friendly. Maybe he is a visual person and this is kind of like Skype for him or something. His pictures probably hold some body language too. Maybe you both could Skype together, to get to know each other on a more personal level. Kinda improvising for not being there in person.