Overview

This trick might be useful at some point and it’s not very obvious how to do at first. So let’s see how to change a folder’s icon:

1. Open the image to want to use as the icon in Preview and select Edit → Copy to copy the image in the clipboard.
2. Select the folder you wish to customise, then Right click → Get Info.
3. In the top left corner, click the folder image, then select Edit → Paste:

To remove the custom image and restore the default folder icon, in Step 2 above select Edit → Cut. Simple as that!