This can be a bit tricky if you are use to IE and not firefox, so here is a short walktrough how to do it.

1. click on your name in the login box, when you are logged in
2. press your account
3. Point your mouse on the E image
4. in the upper right coner you see a pensil
5. press on the pensil
6. a new page in the middle will be loaded.
7. in the new page press the buttion upload picture
8. browse to the image you wish to upload.
9. when it's fished uploading you avatar to change to the newly uploaded