Most of my photos are public, but not all. If you want to be added as a friend, you need to meet two requirements:

1) You have to have pictures on your own flickr page. If you want to get, you gotta give as well.

2) They have to be pictures I'm interested in seeing. Sorry, but there's a lot of stuff I don't particularly want to see when I'm browing through the latest photos from my contacts.

That's all.