One idea is to take pictures of girls you've already have had sex with. Once she's let you enter that private area, taking pictures of her in the bathroom seems milder. Just follow her when she needs to empty her bladder after sex, or pee out the cum. Not having a lock on your door could help.
I know a guy who takes pictures of all the girls he has over at his house. He's shown me pictures of seven girls sitting on his toilet and smiling, all of them hot. Unfortunately one of them includes my girlfriend. He made her visit him, took her virginity, and photographed her while she' was peeing out his cum. The picture shows her smiling insecurely at the camera from the toilet seat, just wearing a blouse with nothing underneath. Man it made me angry to see that, although I knew what happened.