Sorry for two updates in two days, but a couple users—including me!—noticed an issue in Usher 1.1.11: Changes to the library weren’t sticking between relaunches, and Usher was recreating thumbnails on every launch.
You’ll only have this problem if you keep your main Usher library file on an external volume (which isn’t recommended, but some of us live on the edge!). The problem was that Usher’s path determination for these library files was failing, leading to a failure to write the library file on quit.
Usher 1.1.12 fixes that problem, and is available now via in-app updates, or by downloading the new version from our site. Sorry about that, and thanks to everyone who reported it!