PNGcrush damages nothing. It uses libpng to reduce images to their minimal size, when you use the -brute -reduce options.
You have the same image, same quality, same colors, no difference at all when you see it. But it has the *good* size.
PNGcrush's author is one of the libpng's author. He knows what he's doing.
To use it, just open a dos command window, cd to the dir where you dezipped pngcrush.exe, and type pngcrush -help
It'll explain you all you need to know.
You can also use it through Cygwin (much better
or from an unix/linux box.
By the way, PNGcrush 1.5.8 is out : http://sourceforge.net/projects/pmt/