You can easily change any icon folder from any picture you want (copy the picture you want -> get info of the folder -> click the actual picture folder -> paste the new picture).
You will also need to set your dock folders appear as a folder and not as a Stack : right click on the folder in the dock to pop up a list and choose make it appear as a folder and not as a stack (showing the last item you put in it).
To do so :
- Hold your mouse over a stack and right-click on it until a menu appears
- Once that menu appears select “Display as” and choose Folder.
From Apple support to change picture of folder :
Create custom icons for files or folders on Mac
You can create a custom icon for any file or folder using your own pictures, icons downloaded from the web, or the icon from another file or folder.
Use your own picture or a picture from the web
On your Mac, copy the picture you want to use to the Clipboard.
One way to do this is to open the picture in the Preview app , choose Edit > Select All, then choose Edit > Copy.
Select the file or folder whose icon you want to replace, then choose File > Get Info.
At the top of the Info window, click the picture of the icon, then choose Edit > Paste.