jimmykitten.user.js

// ==UserScript==
// @match http://*.wikipedia.org/wiki/*
// This was written by Etienne Millon <etienne.millon@gmail.com>
//
// Replace Jimmy Wales' "donate now" pics by kittens.
// ==/UserScript==

function replaceJimmy() {
  var img = $("img[alt='Jimmy Wales']")[0];
  var url = 'http://placekitten.com/'+img.width+'/'+img.height;
  $("img[alt='Jimmy Wales']").attr('src', url);
}

window.onload=replaceJimmy;