When you go to UserCp you should get the Edit Avatar in the left hand box and from there the editing options unless you have the "do not use an avatar" button on or the "use custom avatar" button off (green=on).
If its not appearing at all check Internet Explorer, Tools, Internet Options - security, privacy and advanced then try resetting them to the default levels if they are set very high.
Various firewalls, anti-virus and parental control softwear can sometimes prevent odd parts of web pages from appearing but if your using Firefox I hav'nt a clue.