Ok, I'll give it a go. Firstly all u need is an image. Any image, one u like preferably and one that doesnt make u look like a schmuck...
Then u save this image on your machine. Login to SAB, and go to your profile - you can find the link on the top right of the SAB pages. Once your profile comes up, look for a text box with a browse button next to it. Click the browse button and browse to your image - select it by double clicking or choosing open. Then update your profile by clicking submit. And Booyaa you got ur image.
Bear in mind the following: From what Ive seen AB are using some sort of compression logic, and being that the site uses .net, its probably GDI+. This means that if you upload an Avatar that is really big, the compression will actualy make it smaller, to fit in the 80x80 pixel requirement on the page - this means that if your image doesnt start out being a perfect square, it will distort - also if your image is very big (and of a very high quality) some quality loss will occur.
The best way to go about this is to create a new image with size 80x80 pixels in an image editing software package like photoshop - then save it on your machine and use that image - this way you wont get any quality loss etc. You can also use transparent gifs and animated gifs on the site - they work. STAY AWAY from png formats as they arent supported in IE6 - so some peeps wont see ur cool new avatar...
Any other questions? Ask away Ill try and help.