I don't know what the actual requirements for the forum avatars are, but I recommend no larger than 100x100 pixels, and either .jpg or .png format (I know both of these work). I don't know if .gif format will work or not, it may.
Also, some of these steps may not be totally necessary, but I think it *should* work at least:
click on your username where it says "Signed in as _____" to go into the setup.
click on the "Avatar" tab
- click the radio button to the left of "Yes" for "Enable Avatar"
- click the "Save Changes" button at the bottom
it should reload the page and you should still be on the "Avatar" tab
- click the "Browse" button next to "Upload Avatar"
- locate the file on your harddrive and click "Open"
- click the "Update" button next to the "Browse" button and wait a moment while it loads (I think the whole page reloads, like when you clicked Save Changes before)
I don't know if it is necessary at this point, but you might want to just click "Save Changes" at the bottom again. Then y'gotta wait a little while to see if it updates. As it says up at the top, it could take up to 10 minutes.
A couple ways to check are either to look at a post you've made, as it should show your new avatar there (easiest, imo), or to go back into the setup page, as it should show there next to your name, also.
Hope this helps,
//Mary (nissus)