Just a few moments ago, the new avatar/icon system has been activated. This means you can now do a lot more with your avatars than simply upload one and overwrite it again when you want to have a new one.
What's new then?
There is a new page, which can be found via your profile (follow the links on the upper right side of your window). This page is your avatar manager.
You can upload a maximum of ten images as your avatar that are smaller in 'weight' than 10kB. The requirement that images have to be exactly 60x60 pixels has been dropped because it was unclear to the users. Just be aware that icons that are not square can be displayed in a (slightly) other way than you can expect. You can preview that in your avatar manager
You can select one of the uploaded images to be used on the forum, leaving the others in your manager, without the need to delete them.
You can also delete avatars you don't need, so you are able to upload new ones if you already had ten.
I'm sorry, what on earth is an avatar?
An avatar, to be short, is the image displayed under your username when you are active on this forum. When changing the current avatar, all the avatars belonging to posts that were already in the forum, will be changed to the new one right away.
More information in the forum's FAQ