Exmh was complaining about libpng not being the same version as what was used to compile the program. Since exmh is pretty old, this is not unusual. The package that we needed was libpng10, here is the description:

The libpng10 package contains an old version of libpng, a library of
functions for creating and manipulating PNG (Portable Network Graphics)
image format files.

This package is needed if you want to run binaries that were linked dynamically
with libpng 1.0.x.

Unfortunately, this package is not included with RHEL5. (It’s only in RHEL4 and RHEL3.) I did find a copy on the web and installed it so that exmh no longer complains about the images.

The rpm is saved in /system/kickstart/install/additions.