I think it's because the the forum scales the avatar images to 90px wide, but the space for the avatar in the layout is 150px wide so the browser scales up the image to fit the space. Because the image is scaled up from a smaller copy it looses quality so they all look blurry.
If there is a setting in the forum to scale avatars to 150px wide instead of 90px it will make them look crisp and not take much additional bandwidth.
You can see this if you right click on the avatar image and open the image itself. It's smaller than what it appears on the forum.
Hope that helps.