Sigil

• http://code.google.com/p/sigil/
• http://lwn.net/Articles/456729/ Ebook editing with Sigil
• http://youtu.be/py5-AwmCP_0 How to Create a Master Ebook Template using Gutenberg and SIGIL

• http://www.mobileread.mobi/

' http://portables.about.com/od/ebookreasers/a/Ereader-Sideloading.htm

Again, my big thing is cross-platform compliance, and that's where I'm running into trouble. 
Putting your image in an SVG "wrapper" will let you zoom it in and fill the screen. 
That code is pretty straightforward:
<svg xmlns="http://www.w3.org/2000/svg" 
xmlns:xlink="http://www.w3.org/1999/xlink" 
version="1.1" 
width="100%" 
height="100%"  
viewBox="0 0 497 765" 
preserveAspectRatio="xMaxYMax meet">
     <image width="497" height="765" 
xlink:href="images/frontcover.png"/>
</svg>