gistfile1.js

// ==UserScript==
// @name           Tumblr Auto Image Expansion
// @namespace      http://twitter.com/uu59
// @include        http://www.tumblr.com/*
// ==/UserScript==

(function(){
  function auto_image_expansion(target) {
    var doc = target || document;
    var img = doc.querySelectorAll('div.post_content p img.inline_external_image');
    var click = unsafeWindow.document.createEvent('MouseEvent');
    click.initEvent('click', false, true);
    Array.prototype.forEach.call(img, function(im) {
      im.dispatchEvent(click);
    });
  }
  unsafeWindow.addEventListener('AutoPagerize_DOMNodeInserted', function(ev){
    GM_log('AutoPagerize_DOMNodeInserted');
    auto_image_expansion(ev.target);
  }, false);
  auto_image_expansion(document);
})();