When we imported the themes from the old themes.org, most of the screenshots were very large. To save space and bandwidth, we scaled them to a resolution of 640x480. We understand that this makes some of the larger resolution themes quite blurry and hard to see, so we have implemented a new policy of cropping any newly-uploaded theme screenshots to 640x480 from the top left hand corner. Hopefully this is an acceptable compromise between size and attention to detail.