When there are a lot of magnets on your desktop, clear your desktop by folding magnets. When a magnet has been folded, a star will appear and the thumbnails will be hidden. Thumbnails cannot be added to a folded magnet. Drag the star to open folded magnets. Then you are able to add thumbnails to the magnet. Thumbnails can also be easily added to a folded magnet by using the shortcut.

Related commands
Shortcut
Throwing away thumbnails inside a magnet