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": "",
    "sampleText": "Insert comment here"}
 }