Custom icons are stored in the filesystem which means that you can change any drive, folder or file icon as long as it contains/is on a writeable filesystem. 🙂
Just open “Get Info” (command+i) of an object and select the icon in the the top left corner.
Then use command+c to copy, command+v to paste and <– to delete the icon. (deletion resets to default) you can even use a jpeg (just open in preview and use command+a and command+c)