How to Extract & Save High Resolution Icons from Mac OS X Apps

1. Select any app in Mac OS X Finder and hit Command+C to copy to clipboard.
2. Launch Preview and hit Command+N to create "New from Clipboard" (alternatively, use the File menu)
3. Save the app icon as an image of your choice.

How to Apply Icons on Apps & Files on Mac OS X

1. Copy your icon by using Command+C
2. Get info on the target file by selecting it then pressing Command+I or right-click then taping the get info.
3. Click on the existing icon at the top left corner, then paste.