I was checking some old, unresolved bugs. One old problem relates to changing an image outside of jAlbum, and replacing the copy in the image directory. jAlbum stubbornly hangs on to the cached thumbnail (if there is one) - F5 won't update the display in the Explore window. You can delete the image and re-add it, but then you also lose comments, titles, etc. Manually deleting the cached thumbnail works, but is clumsy.
The quick fix in this situation would be to exclude the image, then re-include it. But that doesn't kill the cached thumbnail. It should - there's certainly no penalty for doing so.
Most of them were easy to spot, and the other items that are high on my lists are generally easy to test, too. This one, for example - a lot of the "astronomy" shots that I use for testing don't have embedded thumbnails, so looking for a cached thumbnail to vanish when doing an "exclude" is about a 10-second fiddle.