manifest.json

{ "name": "Tumblr full size images"
, "version": "1.0"
, "description": "Never show image thumbnails on tumblr"
, "homepage_url": "https://gist.github.com/johan/6336414"
, "manifest_version": 2
, "content_scripts":
  [ { "js":
      [ "tumblr-full-size-images.user.js"
      ]
    , "matches":
      [ "http://*.tumblr.com/*"
      ]
    }
  ]
}

tumblr-full-size-images.user.js

// ==UserScript==
// @version     1.0
// @name        Tumblr full size images
// @description Never show image thumbnails on tumblr
// @namespace   https://github.com/johan
// @match       http://*.tumblr.com/*
// @match       https://*.tumblr.com/*
// ==/UserScript==

var changes, _slice = [].slice, changes;
function array(ish) { return _slice.call(ish, 0); }
function $$(sel, ctx) { return array((ctx || document).querySelectorAll(sel)); }
function click(e) { if (e && typeof e.click === 'function') e.click(); }

function update() {
  if (changes) changes.disconnect();
  else changes = new MutationObserver(update);

  $$('img[data-full-size]:not(.enlarged)').forEach(click);

  changes.observe(document.body, { childList: true, subtree: true });
}

update();