Well, I have my own rules when it comes to avatars. As long as they are not used to promote stuff or display something 'nasty', it's their business. I like to have my own 'mug' since I'm trying to build my brand online and make sure people see this face and know it's me, but they can use anything as long as it doesn't break my guidelines.
I have no problem with fictional characters as someone's avatar, but I have to admit that I'm a bit suspicious of those with unrealistic 'real' person photos, because spammers tend to use them on webmaster boards. Especially those who pretend to be experts at 'SEO'.
