Uhm ... how about NOT show her the picture? It's not like it's being posted everywhere for everybody to see. It takes effort to see things like that.
My daughter has no clue this is going on. If she 'happens' to hear something about it at school then I will just explain to her that it was something 'private' that was supposed to be between her and Zach.
And she'll probably say "okay", and that will be the end of it.