diff --git a/lib/cooked_post_processor.rb b/lib/cooked_post_processor.rb
index b60e05a15..9ea2f370f 100644
--- a/lib/cooked_post_processor.rb
+++ b/lib/cooked_post_processor.rb
@@ -130,8 +130,17 @@ class CookedPostProcessor
   end
 
   def get_size(url)
-    return nil unless SiteSetting.crawl_images? || url.start_with?(Discourse.base_url)
-    @size_cache[url] ||= FastImage.size(url)
+    # we need to find out whether it's an external image or an uploaded one
+    # an external image would be: http://google.com/logo.png
+    # an uploaded image would be: http://my.discourse.com/uploads/default/12345.png or http://my.cdn.com/uploads/default/12345.png
+    uri = url
+    # this will transform `http://my.discourse.com/uploads/default/12345.png` into a local uri
+    uri = "#{Rails.root}/public#{url[Discourse.base_url.length..-1]}" if url.start_with?(Discourse.base_url)
+    # this will do the same but when CDN has been defined in the configuration
+    uri = "#{Rails.root}/public#{url[ActionController::Base.asset_host.length..-1]}" if ActionController::Base.asset_host && url.start_with?(ActionController::Base.asset_host)
+    # return nil when it's an external image *and* crawling is disabled
+    return nil unless SiteSetting.crawl_images? || uri[0] == "/"
+    @size_cache[uri] ||= FastImage.size(uri)
   end
 
   def get_image_uri(url)
diff --git a/lib/image_optimizer.rb b/lib/image_optimizer.rb
index 0d8803f7f..30df65d81 100644
--- a/lib/image_optimizer.rb
+++ b/lib/image_optimizer.rb
@@ -90,7 +90,7 @@ private
   end
 
   def optimized_url
-    @optimized_url ||= Discourse::base_uri + "/images/#{RailsMultisite::ConnectionManagement.current_db}/#{file_name(cached_path)}"
+    @optimized_url ||= Discourse.base_url_no_prefix + "/images/#{RailsMultisite::ConnectionManagement.current_db}/#{file_name(cached_path)}"
   end
 
 end