Like someone said before, send a pic of some hunky hot guy nude with a giant donkey dick. She'll get a laugh at least to lighten the situation. Then tell her you won't send a pic unless she does. She's the girl for God's sake physical beauty means more to us than them anyway.
Whatever she sends, whether it shows her face or just her privates or whatever it is. You match it. Don't send her the full monty when all she shows you is her ass or some shit.
If she still doesn't do shit, ask her specifically what she's interested in seeing. She want a dick pic? You can get away with that. Anyone else who sees it you can just deny anyway since it could be anyone's penis. I've been there.