Now, may be found at:
~/.cache/thumbnails/
Directory used as a cache of thumbnails.
https://en.wikipedia.org/wiki/Thumbnail
Standard Specifications:
https://specifications.freedesktop.org/thumbnail-spec/0.7.0/x116.html
Cleaning
You can use the following as a basis to clean thumbnails. You may add this to a cron job.
find ~/.thumbnails -type f -atime +7 -exec rm {} \;
Applications
Various applications use this directory.
gwenview
In Nautilus, See: Edit -> Preferences -> Preview: Show thumbnails.
