    # File lib/compass/sass_extensions/sprites/layout_methods.rb, line 130
            def calulate_vertical_postions
              @images.each_with_index do |image, index|
                image.left = (image.position.unit_str == "%" ? (@width - image.width) * (image.position.value / 100.0) : image.position.value).to_i
                next if index == 0
                last_image = @images[index-1]
                image.top = last_image.top + last_image.height + [image.spacing,  last_image.spacing].max
              end
            end