Avatar is a perfectly good word in use long before message boards or computer games.
It refers to the appearance taken on by someone, particularly a Hindu deity, to be shown to the public. An avatar of the deity Ganesh, for example, is the well known crowned white elephant.
In online gaming, it is the image or persona used to play the game. For example in Lord of the Rings Online my avatar is a short fat white bearded dwarf with heavy armour and a huge axe.
And so, on message boards such as this one, it has come to refer to the little pictures we use to present ourselves to the readers of the board.
Interesting psychological questions are raised thereby, aren't they?