Cerious Software (www.cerious.com) has a (Windows) product called ThumbsPlus (about $150 or so),
which includes batch transformations - I have used it to do batch color transformations, but of
course it also does simple load-save-transformations.
I don't know whether FOP/Jimi handles JPEGs saved by this product - but I think they
have a time-limited trial version for download.
Hope this helps,
Arnd Bei▀ner IT-Engineering
Bahnhofstr. 3, 71063 Sindelfingen, Germany
Email: [EMAIL PROTECTED]