I was going to suggest Doujinshi, but I couldn't find copyright notices on those websites, so I didn't want to risk uploading a copyrighted image OR using a copyrighted character. (for novices, a doujinshi is a manga written by an amateur author, but using well known characters from famous manga, as a sort of tribute.)
What I've uploaded are some old manga images taken from wikipedia and marked as
copyright-free. These are from Hokusai and Kotaro, and by size and proportions they fit our needs.
Also I noticed something
interesting. When I build a Mobipocket format book made of jpgs, the image displayed as "cover" will have no borders, thus using all the screen's space. When one starts reading the next pages, the borders appear.
Maybe there's a way to mark more images with the same "cover" characteristics so they all fit the whole screen.
I'll try with tompe's suggestion, but now I must leave for home.
See you in some hours. Thank you guys.