Also, I just found and tried the MobiPocketUnpacker script from here:
but got errors when when attempting to run it using the drag/drop interface described. I'm using Mac OS 10.6.8. Unless there is another tool for unpacking the MOBI file, it might not be possible to open it after conversion.
The Mobi seems to open and display properly in Calibre, apart from taking a while to load.
I should mention this book is a bit of a monster; in print, it's an 7x10 800+ page legal reference with several appendices made up of placed PDF docs, which accounts for most of those images. Unfortunately InDesign rasterizes all the placed PDF content during EPUB export and saves it out as PNG or JPG images.