New Post: Download image from pop-up lightbox

That's a feature of the web browser, not the lightbox script.
In fact the lightbox script purposefully makes it hard to get the image, but you'll find that some browsers still let you get it with a right-click. Try Chrome.

Note that if you need your visitors to be able to download your images, you can modify the product template to display a direct link to the image/s, The tokens are all in Back Office > Admin > Help.
Regards
Rob