Always remember, IrfanView
is your friend!
I re-saved just images 2 and 3 (quality: 95) and it now works, like JSWolf's solution.
I sometimes have the same problem when converting .prc/.epub ebooks to .imp as the eBook Publisher software I use cannot read some .jpeg's as well. I've never understood why, but just re-saving them with IrfanView has ALWAYS worked for me.
When I wrote my Mobi2IMP conversion software in Perl, I used GD::Image tools to just resave the images extracted from .prc/.mobi ebooks and used those to produce the .imp ebook conversion. Again ALWAYS worked for me!