To finish processing the illustrations I use my go-to image editor: GIMP, http://www.gimp.org
GIMP is Free, open, and runs on any OS. If you are familiar with how Photoshop functions, its fairly easy to understand. Batch processing is achieved through scripting, which has a steeper learning curve than Photoshop action sets, but can be an extremely powerful tool.
First, I opened the 15 illustrations and simply exported them as JPG (the most supported format for EPUB images). This makes images around 1500 x 1900 pixels, for a total of about 8 MB–and the images still look pretty good…
However, the average ereader has a screen size only about 600 x 800 pixels (although newer models are pushing 758 x 1024). Leaving the larger images enables tablets and computers to view the full quality images, but it also makes the EPUB unnecessarily large for ereaders that can’t display it.
I decided to make a second set of scaled JPGs to test in the ebook to see how they preformed on an actual reader. I scaled each image down based on a the approximate size of 600 x 800. This resulted in a batch just over 2 MB.
P.S. I uploaded the full set of JPGs to a new illustration gallery!