There are several free apps that can help you to reduce PNG file sizes on windows or linux. Unfortunately on OS X either the ports are unavailable, non-free or difficult to compile or has a command line interface. It was just random luck that I stumbled into PNGCrusher which is a GUI application to optimize PNG files. Under the hood it uses optipng. Unfortunately PNGCrusher hasn't been updated in some time and the author has no plans to support it in the near feature. The version of optipng used by PNGCrusher is outdated and you may be missing out on some recent features and optimizations if you stick with the version PNGCrusher provides. Hence, I have made a PNGCrusher DMG which has the latest optipng bundled. No guarantees and No promises. Use at your won risk :).
To use PNGCrusher drag the PNG images on to the PNGCRusher application. The app replaces the original file with an optimized file if it can optimize. Since the application rewrites your files it might be a good idea to backup your files before attempting to optimize them.