Help?!

The guy I'm in love with has this thing with his guy friends where they send pictures of other women (in bras and that kinda thing) from offline to one another as like a joking around thing. I have asked him to delete it from his pictures and to stop once before but today i saw it again on his phone. So I told him to delete it again and to stop. How can I explain to him that it hurts seeing that kinda stuff like I'm not good enough or something? I just feel like joking about women isn't right how can I tell him?