I checked in 64-bit support for the PngImage class this week, so
wcs2kml should be able to handle 25k x 25k images now (it converts to
RGBA on read). I didn't make a new tarball, but it's available in the
SVN repository. Try it out and let me know if you find any issues.
http://code.google.com/p/wcs2kml/source/checkout
Jeremy