Avatar has several definitions. I would guess that the context this board is using is an image representing a user in a multi-user virtual reality space. In other words, a graphical representation of yourself that can be added to all posts. Since everyone is an individual, it would only seem fitting that it would be a custom avatar.
Personally, I was hoping that the definition of avatar they were referring to was the manifestation of a Hindu deity, especially Vishnu, in a human, superhuman or animal form. That would have been much more interesting to watch electronically unfold.
Jeff