While this was written for the Kindle, it will likely be useful for other eink devices.
In gearing up for my Kindle 2, I decided to create an a script/action set in Photoshop to easily convert manga (and any other image) to a Kindle optimized version. This Photoshop action/script set will automatically convert your image to greyscale, flip it to portrait mode (if it isn't already), apply a few contrast, sharpening, and smoothing filters, resize and center to 600x800 and finally, export as an optimized .gif file (using 22 colors, just to give the Kindle a bit more to work with -- outputs to c:\Kindle Manga). You may still want to run Mangle to rename and create a book for your files, but UNCHECK any image optimizations in that program if you convert images with this action/script first.
This will work with any image you throw at, but I mostly optimized for manga images with the applied filters! To install, put the two script files in your photoshop/presets/scripts folder and install the action via the action menu. If you have any problems getting it to work, post here!
Rapid share link: XXXXX (see attached)
(I have tested it on my Kindle 2, seems to work great, however, my Kindle suffers from pretty terrible contrast, so it'll be going back to Amazon!
...rather annoying given that I live in Osaka, Japan!)