I use ACD See from ACD Systems. I have both the pc version for my desktop and the moblie version for my Axim.
http://www.acdsystems.com
What's nice is the program includes a utility to resize and send to the Axim via the sync cradle right from the desktop program (you can choose where to store it too internal or external memory). You can also view any images on a sd card or a cf card right out of the camera.
It's my opinion but I think ACD See is the fastest and most versatile image viewer for the desktop, and it works quite well for the Axim too. My only gripe about the Pocket version is that it doesn't default to the SD card when browsing for images (takes two extra clicks) and automatically size 1:1 instead of shrinking to fit. If it would do both of these things, I'd say it was the perfect image viewing software perfect.