the lemming seems to like:

• images with around same height/width, or narrow images, when it comes to thumbnail generation
• for community icons similar idea regarding same height/width, but given circular shape, surrounding opaque or transparent shape may be needed to preserve intended look
• for community banners, short and wide seems most apt to fit whole image
• for sidebars, small square-ish images appear to work best without getting scaled down. tall, narrow images get shrunk and miss their point

specific resolutions would be mentioned, but uncertain of precise relevancy. current arrangements may be examined as reference, however they are subject to change as continued play proceeds.