It could be because the site hosting the picture for your avatar isn't working right now.
It could be because your avatar is too large and the forum won't allow it.
It could be because you typed in an incorrect URL for your avatar and the forum can't show the picture because the URL leads it to an incorrect location.
Other than that, and without knowing more than the fact that it's not working, I don't know what else it is/could be.
