Change the icon of a single, specific file
Open the image that you would like to be the icon in Preview.
Press ⌘A to select the entire image and press ⌘C to copy it.
Get Info (⌘I) on the folder.
Click the existing icon in the top-left and press ⌘V to paste the icon from Preview.


To revert back to the default icon, Get Info on the folder, click your icon and press ⌫.