On Gravatar.com you should go to 'Manage (My) Gravatars' and click on the 'add a new image' link. I've put 'My' in brackets because that word appears in the drop down menu but not on the webpage.
That's the way I successfully change my avatar at least once a day. Did you do something different to that?
Think I've done it guy's Ihad not changed it from X rated to G rated, its now showing on my profile, so hopefully it will be up in a while!!! fingers crossed