simply drag your png into the application and it compress's your image to a lower size and replaces it in the same directory. its real neat.
i tried it on my website which all the png's added up to 3.6mb's - after compression (with no visible quality loss) i managed to get the site down to 2.4mb's
it doesn't seem like much but it could save load times if some of your users are on slow speed connections.
Link to Ping for Mac
i may seem like an advertisement for the application but im not - i dont know the developer or anyone to do with it - its just such a great compression application - beats loading images into photoshop