The "My Stuff" page shows the avatar at 90x90. The small avatars are 28x28.
You should make your avatars square, so that they can be scaled well.
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One of the main sources of slowness of the site is the large avatars and gallery thumbnails. So far we haven't done anything about it but we're about to do a major resizing of all resizable avatars and we are also going to automatically resize new avatars. There might be some loss of quality in the images but it will help us a lot with performance, bandwidth and disk space
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