Hello. I found this java app that could be useful for a Kindle or another ebook reader (like me):
This app converts a batch of images, to a pdf file, and had some options to split, and rotate the images.
Of course, is nothing near Calibre (I will be very happy if it could be ported )
I tested the app in Haiku, with the openjdk package, and works well.
I hope it could be useful to anyone.