No, you don't need to be 1st Class. At the upper right of each forum page is a series of links - Forums My Profile Search Members. Click the My Profile link. You'll be taken to a page where you can view and edit your profile. Click the Edit Profile link at lower left. Scroll down the page until you see...
If you have an Avatar already uploaded to a web server someplace, use Avatar Link: paste the URL into this box.
If you don't have an Avatar already uploaded, use the Upload Avatar link: - click the Browse button and point it to your Avatar.
In either case, your Avatar image will work best and be clearest if you resize it to 80x80 pixels.