That's a good question. I believe it could be because for you to be able to view the picture you'd have to be registered in the forum, which is only allowed to people over 18.
However, if a minor were to come across this website by chance, he/she would be able to view the avatars, even without registring.
Thus, with this rule, we prevent minors (or whoever might come across LSPG unwantingly, if there is such a thing), from seeing mature content.
I could be wrong though. So we'd better wait for an administrator to come and explain =)