I say you take it down. As you said it got you a few extra sign up's so you've made your money from it. Be respectful to the guy and take his girlfriends pic down.
If that was a pic of my girlfriend I would want it taken down as well. Not for the fact she has her boobies out but for the fact she's on a porn site and she is in no way attached to porn (well my girl is coz she owns some sites but you know what I mean) so come on you've had your fun and profit. Be nice
At the end of the day to its your call mate! Make your choice!