Christ is in our midst!
We are not adding icons as avatars because we think that we are those saints (!) but we choose them because we have a special affinity to that particular saint.
In the Orthodox tradition, we are all sacred and holy and we are all icons of Christ Himself! That is why we should see nothing but holiness in another person!
I don't really think that the character of Eastern religious forums would remain consistent to their ethos if avatars of secular characters were allowed--but that is just my humble opinion.