When archiving photos on a backup disk, you eventually will loose all locally stored thumbnail images. They are located in ~/.thumbnails/ and get cleaned up now and then. When accessing the pictures the next time, your file manager has a hard time re-creating all the thumbnails again - which is something you personally don't want eiher, since the time it takes to scale those images is lost time.
Fortunately, the freedesktop.org Thumbnail Specification (mirror) also describes "Local Thumbnail repositories" which may be created per-directory in $folder/.thumblocal/. Now eix did not show me any thumbnail creator, so I was tempted to create one myself. Luckily I searched the web for .thumblocal and found updateThumbnails, a thumbnail creation program.
The tool supports the freedesktop specification and some other ones (XV and size-based directories [like 800x600/, 640x840/ etc])! Beside creating thumbnails, it also keeps the files up to date. What would I want more?