Use dimensions by @sstephenson to add automatic images size and improve browser rendering

dimensions_helper.rb

module DimensionsHelper
  # How this improves browser rendering?
  # https://developers.google.com/speed/docs/best-practices/rendering#SpecifyImageDimensions
  #
  def image_tag source, options = {}
    unless options[:size]
      fs_path = asset_paths.asset_for(source, nil)
      options[:width], options[:height] = Dimensions.dimensions(fs_path)
    end
    super
  end
end