I know this is old, but to anyone wanting to know in the future:
Instructions to set a folder's icon to this on OSX (this doesn't work on windows):
Open up the .png file in preview.
Press Cmd+A to select all of it, press CMD+C to copy
Right click on the folder and click "Get Info"
Click on the folder's icon on the top-left corner of the window so it has a blue border around it
Press Cmd+V to paste the new icon and replace the old one.
To reset the icon:
Open up the Info screen on the folder and select the icon on the top-left corner to select it
Press delete to reset it to normal
Open up finder and go (Cmd+Shift+G) to /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources
Open up the Info screen on the folder you want to reset.
Find "GenericFolderIcon.icns" and drag it onto the icon on the top-left corner of the info screen