diff --git a/lib/sass_extensions.rb b/lib/sass_extensions.rb
index a7d3d450..053a9063 100644
--- a/lib/sass_extensions.rb
+++ b/lib/sass_extensions.rb
@@ -18,30 +18,46 @@ module Sass::Script::Functions
   end
 
   def image_url(path)
-    if absolute_path?(path.value)
+    path = path.value # get to the string value of the literal.
+    if absolute_path?(path)
       return Sass::Script::String.new("url(#{path})")
     end
     http_images_path = if Compass.configuration.http_images_path == :relative
-      if (target_css_file = options[:css_filename])
-        images_path = File.join(Compass.configuration.project_path, Compass.configuration.images_dir)
-        Pathname.new(images_path).relative_path_from(Pathname.new(File.dirname(target_css_file))).to_s
-      else
-        nil
-      end
+      compute_relative_path
     else
       Compass.configuration.http_images_path
     end
+
+    real_path = if Compass.configuration.images_dir
+      File.join(Compass.configuration.project_path, Compass.configuration.images_dir, path)
+    end
+
     if http_images_path
       http_images_path = "#{http_images_path}/" unless http_images_path[-1..-1] == "/"
       path = "#{http_images_path}#{path}"
     end
+
+    if real_path && File.exists?(real_path)
+      path += "?#{File.mtime(real_path).strftime("%s")}"
+    elsif real_path
+      $stderr.puts "WARNING: '#{File.basename(path)}' was not found in #{File.dirname(real_path)}"
+    end
+
     Sass::Script::String.new("url(#{path})")
   end
 
   private
+
   def absolute_path?(path)
     path[0..0] == "/" || path[0..3] == "http"
   end
+
+  def compute_relative_path
+    if (target_css_file = options[:css_filename])
+      images_path = File.join(Compass.configuration.project_path, Compass.configuration.images_dir)
+      Pathname.new(images_path).relative_path_from(Pathname.new(File.dirname(target_css_file))).to_s
+    end
+  end
 end
 
 # XXX: We can remove this check and monkeypatch once Sass 2.2 is released.