If a project contains PNG images, and jAlbum is set to Force JPEG images, the slide images are reprocessed every time the album is made - new output images are created and uploaded.
Intriguing discovery.... The problem doesn't seem to occur if I choose bothForce JPEG thumbnails and Force JPEG images. It happens only if only the slide images are being forced, or only the thumbnails. It's the mismatch between them that's triggering the bug.
I think I could make the argument for having a single setting: Force JPEG thumbnails and images. It's tough to construct a reasonable use-case for setting them separately. Maybe I'm missing something....