Under Gnome, it is very easy to change a file icon.
To do this simply:
Right click on the file/shortcut and then "properties".
A window will appear. Simply drag and drop your image onto the icon:
Note: this will work with all file type, folder or shortcut. Practice. However, if you change for example a .doc file or any other specific file type in a folder, only the selected file will have your image icon.
To change the icons of all the .doc files of system, we must change the icons in the folder** /usr/share/icon/