filter_tag.js

// note: if you don't need IE8 support, you can just do
//     return image.tags.indexOf('TAGNAME') !== -1;
filter: function(image) {
  var foundTag = false, tags = image.tags;

  for(var i=0,len=tags.length; i<len && !foundTag; i++) {
    if (tags[i] === 'THE_TAG_NAME') foundTag = true;
  };

  return foundTag;
}