Avatars (not to be confused with the film) are optional images to accompany your user name. I have my BNN (British Naturism News) picture as mine.
To add your's, click the down arrow next to your name (top right), choose
"profile"
then
"edit profile"
Then
"Edit Avatar"
Then you can pick one from stock or upload your own.
Remember, size is important so not too big and not too big a file.
I think 100mb is about right.