In 1.6 SVN, the map previews are new and do look a lot better than before. The problem currently is that the aspect ratio is incorrect.

The area available for the preview is 3:2 on a 4:3 monitor, most people run this shape so 3:2 is the aspect ratio that should be used.

The way I would generate these previews is to do them in a small window with the correct resolution and aspect ratio before hand. Resizing images loses quality so set it to something like 450x300 (3:2) and take screenshots like that.

It would be nice in future if the menu would not stretch it under different aspect ratios which it does currently.