diff --git a/middleman-more/lib/middleman-more/extensions/automatic_image_sizes.rb b/middleman-more/lib/middleman-more/extensions/automatic_image_sizes.rb
index d2039fdd..78a45f3f 100644
--- a/middleman-more/lib/middleman-more/extensions/automatic_image_sizes.rb
+++ b/middleman-more/lib/middleman-more/extensions/automatic_image_sizes.rb
@@ -30,7 +30,11 @@ module Middleman
         # @param [Hash] params
         # @return [String]
         def image_tag(path, params={})
-          if !params.has_key?(:width) && !params.has_key?(:height) && !path.include?("://")
+          params[:supported_extensions] ||= %w(.png .jpg .jpeg .bmp .gif)
+          
+          if !params.has_key?(:width) && !params.has_key?(:height) && !path.include?("://") &&
+            params[:supported_extensions].include?(File.extname(path).downcase)
+            
             params[:alt] ||= ""
 
             real_path = path