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...
Avatar Link:
OR
Upload Avatar:
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.