But explaining the OP's position on photograph sharing could just open up a lot of arguments by the friend as to why her position is wrong. Anytime you offer an explanation, you open yourself up to a lot of nit-picking about why your position is wrong. It is sometimes the safer course to state your position without an explanation. "I'm sorry, I don't share my pictures. But for you, as a friend, I'll give you the pictures that you are in."
I'm a bit surprised that someone would break off a friendship over pictures. If I ask a friend for copies of pictures or a recipe or something similar, things that the majority of my friends would share without hesitation, I admit that I'm startled to get a "no" in response.
But I wouldn't break a friendship off over that "no." Instead, I'd realize that this friend has different opinions about sharing pictures or recipes. And that's something I just have to accept in order to maintain the friendship, just as the friend is probably accepting certain aspects of my behavior that aren't exactly in line with his/her feelings and beliefs.
Now, if the not-sharing was about more than just pictures or recipes, or if there were other aspects of the friend's behavior that started to bother me--let's say the friend never chips in gas money when being given a ride, never tips at restaurants, never shares anything, spreads mean gossip, etc.--then the not-sharing of pictures would become one part of a large set of reasons why I'd break off the friendship.
But to break off an otherwise good friendship due to one quirk of not sharing photographs seems a bit harsh to me.