Jekyll plugin to generate thumbnails from a directory of images and display them with a Liquid tag

0. Install ImageMagick and the RMagick gem (Here is good instruction how to Install rmagick on OS X Mountain Lion).
1. Drop galleries.rb into your Jekyll site's _plugins folder.
2. Add the following to your _config.yml and customize to taste.

gallerytag:
    source_dir: images/gallery
    destination_dir: images/thumbs
    thumb_width: 150
    thumb_height: 150
    columns: 4
    custom_attribute_name: data-lightbox

• source_dir — The path (relative to your top Jekyll directory, where is _config.yml stored) to the folder containing your gallery images. Default value images/gallery
• destination_dir — The path to you thumbnails (relative to your top Jeyll directory). Recommend to use a different path in relation to source_dir to have possibility easily remove all thumbnails in one time. Default value: images/thumbs
• thumb_width — The width, in pixels, you want your thumbnails to have. Default value: 150.
• thumb_height — The height, in pixels, you want your thumbnails to have. Default value: 150.
• columns — How many columns galleries should display when the Liquid tag is used. Default value: 4.
• custom_attribute_name - add into "a" tag custom attribute with specified name which contain the "galleryname" (userful for lightbox plugins, when need to tag). By default custom attribute name is rel for backward capability with old version of JekyllGalleryTag.

Jekyll will automatically generate (during builds) thumbnails for any images in the folder specified in source_dir variable of _config.yml and put them into destination_dir. To display them in a post, you would use a Liquid tag set up like this:

{% gallery galleryname %}
subfolder/myfirstimage.jpg:: A caption!
subfolder/myseconfimage.png:: Another caption
subfolder/mythirdimage.jpg
subfolder/myfourthimage.png
subfolder/myfifthimage.jpg
{% endgallery %}

subfolder/ is a directory with images in you source_dir.

Jekyll will output some HTML that is (intentionally) similar to what WordPress does for galleries in posts, making it relatively simple to tweak your CSS. It will also add custom attribute (default - rel) to the links, which contain the "galleryname" text as shown in the above example. This makes is easy to integrate a lightbox script like FancyBox or Lightbox2.

You can see it in action on my personal blog, here.