It's a known problem, without a known solution. It's actually not a Gossamer Forum problem, but a browser problem.
Specifically, when you upload a new avatar, the avatar _is_ properly uploaded. Now, when you view a page displaying your image, Gossamer Forum sees the new image stored on the server, and properly returns the new dimensions of the image. But, when the browser sees a request for, i.e., /pics/gforum/users/user_icon-12345.gif, it realizes that it already has that image in its cache, and doesn't bother to request it from the server. You can fix the problem for your display by doing a refresh on one of the pages displaying your image (holding down Ctrl when pressing refresh might be necessary).
2.0.0 will fix this by adding some sort of variable (perhaps unix time) to the filename, so that new images really are new images to the browser as well as Gossamer Forum.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com