To add an avatar, first get an 80 pixle by 80 pixle gif or jpeg image to use. It can be smaller, but not bigger (mine is exactly 80 x 80 if you need a guide.
Once you have your image, go to your account options here, using the side menu (your account), or the options button in the upper right hand side of most forum pages.
Once there, click on the "your files" section. using the tool there, click browse, find your image, then upload it.
Once it is uploaded, go back to the main options page. you should see a drop down menu with your image file name. Pick it, then you are all set.