I thought I'd throw a little something out here for fun about avatar sizing after searching through gobs of uk-yankee posts and not quite seeing this:
I experimented around with uploading different images for the avatar, trying differerent sizes and resolutioons.
After alot of trial and error it hit me to read through the web site code to find where the avatar is getting set, and I saw the image gets resized to 100x100, So... if you care, if you can size and/or crop an image into a little 100x100 pixel box, the code should display it correctly (not too squished, not too stretched). It didn't seem like the resolution was as important. Screens typically display at 72dpi, but this image I uploaded was 300dpi (at 100x100) and it seems to be fine.
For what it's worth. That and 3 quid will get you a cafe latte at Starbucks.