I'll try to give you some instructions for selecting an avatar. You might want to copy the list to text editor or otherwise open a new browser window before you follow the links in the instructions, since you will of course lose the list if you follow a link in this window.
1) Edit your profile by Clicking on this link
after you are logged in.
2) You should see a long web-form, starting with a section called "Registration information". Scroll to the bottom of this page, until you see a section called "Avatar Control Panel"
3) Click on the "Show Gallery" button in the avatar control panel
4) A screen with picture thumbnails should appear. Beneath every picture is a clickable circle. Click the circle under the picture you would like to use as an avatar. The circle should be filled in afterwords.
5) Click the "Select avatar" button at the bottom of the screen
6) You should return to the profile editing form. Your selected avatar should appear in the control panel. If so, click the Submit button at the bottom of the form.
If you have a custom picture that you would like to use as an avatar, but it is too big to upload, and you don't have software to change it's size, you could PM me and I'll tell you how to email it to me and I will get it setup for you, if you like.
Hope this is useful,