Well, as I said, it shouldn't happen again - what seems to have happened is that one of the posters on that page has lost his domain registration, and the avatar image was on the old domain. Whoever now controls the domain has tweaked the settings - the pop-up was coming from that server, not ours. It's not a porn site or anything (I braved the link to see), I think the strange message about Tom's Pretties is some sort of password reminder.
In any case, I removed the avatar so there's no reason for it to reoccur.