I too use ImageMagick. Its quality is excellent, and it handles a huge variety of image formats (e.g. PSDs, PhotoCD...).
You'll have a hard time tracking down PyMagick. However, as Shane has pointed out, is there a compelling reason for using python wrappers around native code anyway? If you use popen2 or similar modules, you get to start the conversion in a separate process, and connect to its input/output/error streams using pipes. With (potentially) intensive processes like thumbnailing a Photoshop image of unknown size, aren't you better off doing it in a separate process anyway? You could even farm off conversion routines to a separate box.
If you want to get really whacky, I happily used a python wrapper, by using pyperl around the perl one. Worked fine for me and the perl module is more complete than than the python one, performance was probably not the best though :) Cheers. -- Andy McKay.