There are two basic ways to install pngcrush on RHEL 5. From RPM or from source. If you are able to use non Red Hat repositories I would recommend using DAG. However if your system policy to only allow Red Hat repositories pngcrush you need to install from source. It is very easy and painless.
sudo su - cd /usr/src wget http://downloads.sourceforge.net/project/pmt/pngcrush/00-1.7.12/pngcrush-1.7.12.tar.bz2?r=http%3A%2F%2Fsourceforge.net%2Fprojects%2Fpmt%2Ffiles%2F&ts=1285706068&use_mirror=voxel tar -xjf pngcrush-1.7.12.tar.bz2 cd pngcrush-1.7.12 make cp pngcrush /usr/bin