for those that dont know, you cant actually take the url for the avatar off the site when ur done, it blanks out and wont work.
i struggled with it and then i remembered that i had to get a photobucket.com account (free) so i could put an avatar on my guild page for world of warcraft.
so, what i did was, saved the avatar image to my computer and uploaded it to photobucket.com and copied the direct link, and pasted that into the space between the two (insert here) [img] for the signature.
for the avatar, i took the direct link again and pasted that into the link off-site part i also maxed out the number of pixels dimensions u could have on the bottom 90x90.
worked for me so im happy :P