Long story short:
Apple saves the thumbnails that you see with quicklook, even if you delete the original.
If you see the images stored into an external drive and then unmount it, you might think, “hey I am safe”. Well, apparently you are not.
So you have two options:
a.
rm -rf $TMPDIR/../C/com.apple.QuickLook.thumbnailcache
Then sudo reboot.
b. execute the
qlmanage -r cache
command, which appears to purge the cache without requiring a reboot.