Add Ace::ImageThumbnailerFilter

in haml template use
	%thumbnail{src: "/images/image.png"}

or with defined thumbnail size
	%thumbnail{src: "/images/image.png", size: "20x30"}

lib/ace/filters/image_thumbnailer.rb

@@ -0,0 +1,39 @@
+# encoding: utf-8
+
+require "ace/filters"
+require "nokogiri"
+
+module Ace
+  class ImageThumbnailerFilter < Filter
+    def thumb_name(filename)
+      filename.gsub(/\.([^\.]*)$/, '_thumb.\1')
+    end
+
+    def thumbnail_nodeset(filename, doc)
+      a = Nokogiri::XML::Node.new 'a', doc
+      img = Nokogiri::XML::Node.new 'img', doc
+      a.set_attribute('href', filename)
+      img.set_attribute('src', thumb_name(filename))
+      img.parent = a
+      a
+    end
+    
+    def make_thumbnail_image(filename, size)
+      size ||= 20   # default size
+      cmd = "convert content#{filename} -resize #{size} content#{thumb_name(filename)}"
+      warn "~ make thumbnail with '#{cmd}'"
+      system(cmd)
+      raise "Error when converting image 'content#{filename}'" if $?.to_i != 0
+    end
+
+    def call(item, content)
+      puts item.inspect
+      doc = Nokogiri::HTML(content)
+      doc.css("thumbnail").each do |thumb|
+        make_thumbnail_image thumb[:src], thumb[:size]
+        thumb.replace thumbnail_nodeset(thumb[:src], doc)
+      end
+      doc.to_s
+    end
+  end
+end