Given the path for an image in the local file system, returns a data URI string
representing the data in that image file.
function(localImagePath) {
        // Read the file in and convert it.
      var imageFile = fs.readFileSync(localImagePath);
      var mimeType = mime.lookup(localImagePath);
      var ret = "data:";
      ret += mimeType;
      ret += ";base64,";
      ret += imageFile.toString("base64");
      return ret;
  }