Recently we refactored all of our preview themes. In the past a lot of merchants complained that images are cropped when creating previews. So we changed that.
From now on every preview theme has a note e.g. Best for 3/2 aspect ratio so you know you should use this for previews created by images of a cam. You should use Best for 16/9 aspect ratio for previews made of images from a video filmed in 16/9 format and so on.
Remeber you can always propose new previews, too!