The .png files you have will need to be converted to windows .ICO files before they can be used.
To do this i can recommend
IcoFX a freeware editor which is compatible with the 256 pixel .ico format required by Windows 7.
Once you have the required icons right click on the pinned item and then right click on the program name and select properties - here you can select change a Icon and browse to the files you have created