Topic, Level "Space Adventure", game "Ringmania 2.5"

  You'e right, but right now there is not an api for this, and it's not so quick to do.
The easier way is maybe to add a warning directly in the editor, why not when you open the backgd editor.

  Perhaps you should code a feature that automatically captures the background image as the thumbnail? I suspect that this will be a big problem when the game goes public.

  The ship is not symetric.
The background is not very neat.
As we decided in the comments of the game earlier, the thumbnail should be the capture of the background image (when you edit it).