iphone 3gs not receiving photos?

[Brigitte<3]

My friend has been trying to send me a photo onto my iPhone. It was working before, but recently when she tried to send a photo it never showed up. I am able to send photos to her but she cannot send photos back to me. She tried on her friend's phone as well and that did not work. Is this a software or network problem?
Any help would be appreciated, thanks.