Normally you put the Avater in the left, because you read from left to right and thus you expect to get the first informations you need to unterstand the conten on the left. That is why most forums have their Avatars or userpics on the left, so you can always see who is saying what within you natural field of view before reading the text. It can get irritating for some, and I still catch myself looking on the wrong side sometimes… but to be honest, in the end it all boiles down to a personal perference of layout. I have no real problem with this. Its not that I don’t care, its just that it really doesn’t bother me that much. I feel that you have to decide what you like best, all that science that I wrote on top is just the given reason why placing the userpic on the left is so popular.
With PHPBB2 you can have serveral layouts and let each individual choose for themself, that way you can really satisfy everyone .