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 or
here. 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. The page for that plugin can be found
here.
The program can also be run via Mono on Linux and OSX.