Why can't I receive a picture message?

Okay so I have iPhone 5 on AT&T and my friend who I think is on Verizon (not sure) tryed to send me a message and I never received it? I don't know why I mean I can get them from other people but never him… He tryed multiple times and I still never got it… Does anyone know how to fix this I need help… Please and thank you so much:)

On setting
go on messages
turn imessage on