Show PNG transparency in Lightbox with light background

In Sparkle Pro V4.0.7 I have added a PNG image with transparency and chose “Zoom Image in Lightbox”. The background of transparency is always dark even when I choose the setting “Background = Light”. Is there a way to show the transparency of a PNG image in the Lightbox with light background?

Thanks for all tips,

The option to set the background dark or light did not mean the background of the image itself.
It means the background behind the lightbox, so it looks separated to the website.

You can achieve something like that by using a popup instead of the show in lightbox.