Liquid Image Tag for Jekyll

better_image_tag.rb

# A simple plugin for Jekyll that allows you to use {% url "alt text" %} to add images to your posts. 
# It will automatically check those images for width and height.
# 
# Requires http://imagesize.rubyforge.org/

require 'image_size'
require 'open-uri'

module Jekyll

  class ImageTag < Liquid::Tag
    @img = nil
    @alt = ""

    def initialize(tag_name, markup, tokens)
      @img = markup.strip
      if temp = @img.match(/^(.*)(\s)(\")(.*)(\")$/)
        @img = temp[1]
        @alt = temp[4]
      end
      
      super
    end

    def render(context)
      w,h = open(@img, "rb") do |fh| 
        ImageSize.new(fh.read).get_size
      end
      
      if @img
        "<img src=\"#{@img}\" width=\"#{w}\" height=\"#{h}\" alt=\"#{@alt}\" >"

      else
        
      end
      
    end
  end
end

Liquid::Template.register_tag('img', Jekyll::ImageTag)

Hello, when using this, I've had success if you invoke the tag

{% img url "alt text" %}

for the updated header, see https://gist.github.com/alvarogarcia7/9aebc6ca3db08a26d6378e5bc4611a96