There is a built-in feature in the Layer Slider to have an image show enlarged in a lightbox. This can be useful if you have more images in the slider, or even if you also would like to show that in larger size.

How to enable this feature?

You only need to go to Link tab in the layer settings and add the full URL of the image, which you would like to show in the lightbox. Then under the textfield, you need to select the Open in gallery value for the selectbox.
You can check it during working on the following link:

Popup gallery settings
Popup gallery settings