Android Application sending picture + data between phones through server

So I'm trying to figure out how to implment my thoughts.

I want my user to take a pic with the camera, and send it to another phone.

I've figured out that I need a server and a web-servic. I've read about REST.

But with what I'm struggeling is, how can I upload a picture through REST? Or are there any other services I should use for this? (I also thought about a socket connection between the phones but I dont think this is a good alternative) And If I have the picture on the server, how do I send it to the other user? (Maybe he hasnt opened the application? or has no internet connection?)

I know these are some basic questions, but I would be happy to get some experienced input.