[generateImageString description]
@param  {[type]} cssFilePath [description]
@param  {[type]} fullUrl     [description]
@param  {[type]} imageUrl    [description]
@return {[type]}             [description]
function(cssFilePath, fullUrl, imageUrl, options) {
    imageUrl = imageUrl.trim()
      .replace(rQuotes, "")
      .replace(rParams, ""); // remove query string/hash parmams in the filename, like foo.png?bar or foo.png#bar

    // See if this image was already processed before...
    if(cache[imageUrl]) {
      // If we already processed this image, just pull it from the cache.
      return cache[imageUrl];
    } else {
      // If this is a novel image, encode it as a data URI string and then cache it.
      var loc = imageUrl;
      var isLocalFile = !rData.test(imageUrl) && !rExternal.test(imageUrl);

      if(!isLocalFile) {
        // Ignore non-local image references.
        return fullUrl;
      } else {
        // local file system.. fix up the path
        loc = imageUrl.charAt(0) === "/" ?
          loc :
          path.join(path.dirname(cssFilePath), imageUrl);

        // If that didn't work, try finding the image relative to
        // the current file instead.
        if(!fs.existsSync(loc)) {
          loc = path.resolve(__dirname + imageUrl);
        }
      }

      var encodedImage = getDataURI(loc);

      // If the encoded image meets the criteria for maximum image size, return its data URI.
      // Otherwise, just return the original image reference.
      if(encodedImage != null) {
        if(options.maxImageSize === undefined || encodedImage.length <= options.maxImageSize) {
          var url = 'url("' + encodedImage + '")';
          cache[imageUrl] = url;

          return url;
        } else {
          console.log("The image " + imageUrl + " exceeds the specified maximum data URI size of " + options.maxImageSize + ", so will not be converted.");
        }
      }

      // If the image exceeds the maximum data URI size (or otherwise failed to encode),
      // fall back on the original URL reference.
      return fullUrl;
    }
  }