Mordicus:monobook.js

 
if (mwCustomEditButtons) {
 
   mwCustomEditButtons[mwCustomEditButtons.length] = {
     "imageFile": "http://images.wikia.com/central/images/7/74/Button_comment.png",
     "speedTip": "Comment visible only for editors",
     "tagOpen": "<!-- ",
     "tagClose": " -->",
     "sampleText": "Insert comment here"}
  }