Avatar size is handled strangely when using an url

[juhakall]

I just changed my avatar to a 80x80 PNG image, but it didn't work correctly at first. When I used my Gravatar URL, my avatar was stretched to 120x80 by the HTML, even though the original image was 80x80 when I examined the stretched image. After downloading the PNG from the Gravatar URL myself, and uploading the file as my avatar, it wasn't stretched anymore.

Is this a bug, or some feature I can't understand?