Produces an array of background-image urls used in the page

getBackgroundImages.js

/* Builds a list of background image urls from inline styles / linked stylesheets
 *
 * @return {array}
 * Array of unique absolute image URLs from inline styles/readable style sheets
 */
function getBackgroundImages(doc) {
  doc = doc || document;
  var sheets  = doc.styleSheets
    , url_re  = /\burl[(\s]+['"]?([^"')]+)["']?[\s)]+/g
    , _slice  = Array.prototype.slice
    , bgImg   = style('backgroundImage')
    , getUrls = allMatches.bind(url_re)
    , uniq    = {}
    , urls    = []
    , sheet, rules, rule, selector, bg, match, rel_url
    ;

  function array(ish) { return _slice.call(ish, 0); }
  function style(c) { return function(r) { return r&&r.style&&r.style[c]; }; }
  function resolve(url, base_url) {
    var old_base = document.querySelector('base')
      , old_href = old_base && old_base.href
      , doc_head = doc.head || doc.getElementsByTagName('head')[0]
      , new_base = old_base || doc_head.appendChild(doc.createElement('base'))
      , resolver = doc.createElement('a')
      , real_url
      ;
    new_base.href = base_url;
    resolver.href = url;
    real_url = resolver.href;
    if (old_base) old_base.href = old_href;
    else doc_head.removeChild(new_base);
    return real_url;
  }

  // add all inline background-image style attributes
  array(doc.querySelectorAll('[style]')).forEach(addBGUrls);

  // add all background-image rules used on page from stylesheets
  for (var s = 0; sheet = sheets[s]; s++) {
    try { rules = sheet.rules ? sheet.rules : sheet.cssRules; }
    catch (e) { continue; } // avoids cross-domain exceptions
    if (!rules) continue;

    for (var r = 0; rule = rules[r]; r++) {
      if (!(bg = bgImg(rule)) ||             // not a background?
          !(selector = rule.selectorText) || // not a selector rule?
          !doc.querySelector(selector))      // not used in this page?
        continue;
      url_re.lastIndex = 0; // start scanning from start of rule
      while ((match = url_re.exec(bg))) {
        rel_url = match[1];
        uniq[resolve(rel_url, sheet.href)] = 1;
      }
    }
  }

  for (url in uniq) urls.push(url);
  return urls;
}

// sloppier version that doesn't uniquify, resolve or filter for used urls:
function getAllBackgroundImages(doc) {
  var _slice  = Array.prototype.slice
    , bgImg   = style('backgroundImage')
    , urls    = []
    , url_re  = /\burl[(\s]+['"]?([^"')]+)["']?[\s)]+/g
    , getUrls = allMatches.bind(url_re)
    ;
  function array(ish) { return _slice.call(ish, 0); }
  function style(c) { return function(r) { return r&&r.style&&r.style[c]; }; }
  function allMatches(s) {
    var res = [], got;
    this.lastIndex = 0;
    while ((got = this.exec(s)))
      res.push(got[1]);
    return res;
  }
  function addBGUrls(of) {
    urls.push.apply(urls, getUrls(bgImg(of) || ''));
  }

  doc = doc || document;

  // add all inline background-image style attributes
  array(doc.querySelectorAll('[style]')).forEach(addBGUrls);

  // add all background-image urls found in stylesheets
  array(doc.styleSheets).forEach(function(ss) {
    array(ss.rules || ss.cssRules || []).forEach(addBGUrls);
  });
  return urls;
}