Quicklook registers com.apple.quicklook.ThumbnailsAgent XPC service that is responsible for creating thumbnails database and storing it in /var/folders//C/com.QuickLook.thumbnailcache/ directory. It means that all photos that you have previewed using space (or Quicklook cached them independently) are stored in that directory as a miniature and its path. They stay there even if you delete these files or if you previewed them in encrypted. containers.”]
Source: https://wojciechregula.blog/your-encrypted-photos-in-macos-cache/