Mine is one of my goats. The picture is 10 years old. He hasn't changed much. I'd stick with a picture of myself, but the most recent picture I have is about 7 years old now, and I've probably changed a lot, so I hate to use it.
I can remember pictures easier than usernames, and personally enjoy getting to know the people here, as much or more than reading the posts. To do that you have to keep everyone straight. It's hard when there are so many members, so I try not to change avatars often.
:tmi: