ePub Optimizer is a free program from a MobileRead user Toxaris.
Another small tool to help out. This program subsets fonts used in ePUB's (identical to the routines used in my FontShrinker program), but will also optimizes JPG, PNG and GIF images. The compression factor for the ePUB (or actually zip) will be maximized.
Latest version (currently 1.12) can be downloaded from my site. Usage is simple. Create an icon and just drop the ePUB on it or start via command tool.
There is also a Sigil plugin for this program. With that plugin this program can be called from within Sigil to update the images and fonts.
 For more information
- http://www.mobileread.com/forums/showthread.php?t=252011 - MobileRead to discuss windows program
- http://www.mobileread.com/forums/showthread.php?t=252010 MobileRead thread to discuss Sigil plugin.
- http://toxaris.nl/epub-optimizer/ Toxaris web site.