If I straighten a crooked horizon on an image to be used as the theme image of a subfolder, and then drag that image to the folder image, the unedited original becomes the folder image and theme image (with the horizon still askew).
I can select an area with no horizon for the theme image itself, but the folder thumbnail remains obstinately askew.
Using Jupiter skin (but I don't think this is a skin issue).
Have I missed an option in the edit page that would correct this issue?

OK, thanks for that.
I can understand the problem and can get around it when the photographer sends me more photos. This is a "work-in-progress" and that folder at the moment has only one image.
Or I can stop being lazy and edit the image with Gimp before adding it to the project .

Use the NoPages skin. Set the image bounds stupid large, like 10000x10000 so the image doesn't get scaled. Do your image edits, make the album, grab the output slide image, and use that in your real project.

But the jAlbum core really should take care of this.