Add best compress level for png

transcoder/png.go

@@ -24,7 +24,10 @@ func (t *Png) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, header
 			return err
 		}
 	} else {
-		if err = png.Encode(w, img); err != nil {
+		enc := &png.Encoder{
+            		CompressionLevel: png.BestCompression,
+    			}
+		if err = enc.Encode(w, img); err != nil {
 			return err
 		}
 	}