to answer the original question, i'd say no, there need not be rules in place about avatars. let each choose his own and then let the chips fall where they may. if a person does not like it, they can either use adblock in firefox or turn the avatars off completely. this will allow that person to control his own destiny and not leave things in the hands of the moderating team!
as far as the picture of the girl doing whatever to herself goes, it is currently on my adblocked list. so is that stupid 'look at the face thing' and all the craptacular gifs. if however, poopypants didn't have that picture of batboy jacking off as his avatar, i might not recognize his posts....
by the way, who in god's name is trying to pick up women on a replica watch forum where there are less than 5 (known) female members?
that answer is no one.