Hi, Chris. The one I like best is Resco PhotoViewer. Excellent image reduction algorithm, resulting in very minimal loss of image quality
(minimal in relation to other applications
).
http://www.resco.net/windowsce/photoviewer/default.asp
Another one you may want to take a look ACDsee. There desktop application is very robust - it, combined with ACDSee mobile, would likely satisfy all your needs.
http://www.acdsystems.com/english/products/wince/index
Check them out, take them for a test run ... I like them both, I ended up with ACDsee on my desktop, but Resco on my Handheld. Cheers!