[Highslide] How to set it as default image viewer

Hi, I bought a theme and it comes with some nice gallery template feature.
As you can see in the live demo, in the middle of the left sidebar there are different gallery types which have in common the same “large” image viewer. In fact even if the images are presented in different views (masonry, scroller, flexslider etc) if you click an image from any type of views, the same large viewer is opened: example.

I have already asked help to the author but it has been almost a week without any answer.

Could you please help me to find out how to replace that viewer with highslide?
I already managend on my own how to put highslide into wordpress and make it working thanks to this guide, as you can see from this test page, but not to put it as viewer from the galleries.

In fact if you open a gallery and then click an image, the viewer of the theme is opened, i whish to make open highslide.

Dunno if this information can help: the theme build the galleries views by picking the attachment images of the gallery previusly made in the wordpress admin side. So the images are not published in the gallery text field of wordpress like usual.