Is this good or bad? One possible speedup I could use is to compress more than one image at a time, although the progress indicator would probably have to become asynchronous.

Just curious about PNGOUT - did you get permission to bundle it with your app? Their license http://www.advsys.net/ken/utils.htm#pngoutkziplicense seems pretty restrictive: "Redistributing, repackaging, or reusing the PNGOUT or KZIP executable is prohibited without the express consent of Ardfry Imaging, LLC, and a formal business agreement.

No I didn't, thanks for pointing this out. I'll remove it for now; didn't realize their license had that restriction. That's pretty lame :\.