For two decades it’s been possible to use your own images to customise a Mac’s file or folder icons.
Start by finding a replacement icon. You’ll find loads online, such as
here, or you can use your own image. If using an icon, choose to view the highest-resolution version in GIF or PNG format and download it to your computer (just drag it from the browser to a Finder window).
Double-click to open the picture or icon using
Preview. Select all (Cmd + A, or Edit > Select All) and then copy it (Cmd + C, or Edit > Copy). Close Preview.
Select the file or folder whose icon you want to replace, then tap Alt + Cmd + I (
Option + Cmd + I on some keyboards) to bring up the Inspector (info) window.
See the small icon at the very top left of the Inspector window? Click it once, then tap Cmd + V to paste in your new icon (or click Edit > Paste). Note that this won’t work on a file or folder alias.
The Inspector window will stick around and show details of other file(s) or folder(s) you select. Just repeat the steps above to paste in new icons for them.
To revert back to the default icon, repeat the steps to view the Inspector window and click the icon within it; but this time tap the Backspace or Delete key.
For more information, see:
How to change Mac app icons.