A few reasons. For starters...
1.) In the very first post I said I was waiting for at least 1 tribute to post better pictures. The only tributes I've gotten have been private and therefore those pictures have stayed private.
2.) I actually prefer that her cleavage pictures get tributed, either tank top or bra. Bonus points if you confer with me first regarding side stuff that I'd like to see. I'm not opposed to sharing her topless, but you can see above regarding that.
3.) I've already shared some of her topless pictures in a private chat with another user. He's done 4 tributes already and as a thanks, his username is getting written on her chest for a personal picture just for him. Anybody showing good faith in the fact that it's really my girlfriend will be getting their own forms of proof as well.
I've been in other forums where I've just shown everything and got nothing in return. This strategy seems to be working just fine for me on this site so far