Go to user cp up at the top of this page. On the left of that page will have edit avatar, click on that. Right click and delete the url for the one you have and paste the new one. That should work, if not, come back and post again and maybe mj, jkath or someone that is a site helper can get you an answer.
Pretty good, Texas.
Now, ABJ, once you've deleted your old avatar, you can't get it back. You must place it back. There are controls for uploading a new image, and you can either browse your own computer, or find one on a website. Max you can use is 80x80 pixels, or 64 kb, whichever is smaller.