Perhaps you could try downloading the image onto your computer, and then when setting your avatar, do the file upload option instead of using the URL (ie. not the "http:// ..." one).
I have no reason to think that it would make a difference, but of the few avatars I've seen so far on this thread (Proxima, Pooty, minimac), the image URLs indicates they were all file uploads rather than links.
To download an image, right-click on the image and select "Save Picture As..." if you're using IE. Other browsers should have similar functionality.
[update: I've looked thru a few more people's profiles, and so far my theory is supported. Of the few people that currently uses image links rather than file uploads for avatars, they always get a 128x128 size]
