First, create a user account if you have not already done so.
Navigate to the forum and log in.
In the forum menu on the right side, hit the Profile link. In the upper right corner you should see a little Edit button (if you are properly logged in).
Notice the tabs now in the profile editing area. Click on the Avatar tab.
The third line down in the Avatar editing area is a file browser field. Click on the browse button and choose your avatar from the files on your computer. Your image dimensions must be no larger than 250 by 250 pixels, the file size cannot exceed 50kb, and it must be saved as either a .jpg, .gif or .png.
Press Save. If all went well, you should see your naturally blond avatar there.