If you haven't got an avatar just click on your username on the left and then Modify Profile. Scroll down the page a bit and you'll see a new feature where you can upload your avatar via Image Shack.
Click on Browse to find the image on your computer and then click on Host It.
After a few seconds you'll be taken to a page which shows about five different address codes for your avatar - copy the BOTTOM address (where it says Direct Link To Image) and paste it into where it says Avatar URL in your profile.
It's dead easy.
The same can be done for signature banners. Once it's uploaded paste the bottom code then add the [img code like this:
[img]image address here[/img]
into the Signature box in your Profile.
Please remember to keep your avatars/signature banners to a sensible size like my ones. Any large avatars/banners will be removed!!