Preserve Thumbnails: determine whether to save deleted images' thumbnails or not.
then if we show deleted images by unselecting menu View -> Hide Orphaned Groups and View -> Hide Deleted Images like this:
the result is
All deleted images still have their thumbnail even though the files are moved to recycle bin.
If we turn off this option instead like this:
we will get:
Delete Permanently: if turned on will not move the marked images to recycle bin but immediately delete them instead.