You can easily define the preview image for your masterpage : just go to the Master Page Gallery (e.g. : http://your.server.com/your/root/dir/_catalogs/masterpage/) and find your masterpage in the collection. Then move your mouse hover to have the downward arrow that appears to open its context menu. Choose “Edit Properties”, and you’ll see a Preview Image section 🙂
For your information, the image should 216px × 160px