First of all, that is the colorbox external lightbox jS, not directly Imagevue. Now I am checking your website looking for a link/reference to the colorbox, but can't find any so I can test this. Can you offer me a link to where the issue occurs? Demo version works fine:
Sorry, couldn't see what you were referring to in first post.
Unfortunately, the slideshow filemod is just a simple slideshow between images of the same size in a single element, and does not scale properly like the main gallery sldieshow does. For different size images that may need to scale, you are better of using a normal gallery set to slideshow and removing some navigation elements with folder parameters: https://www.photo.gallery/documentation/folder-parameters/
(Scroll down to the Examples -> Slideshow)