Sticky image gallery from collection

Hi everyone. I have some questions about how to create a sticky image gallery bound to a collection of images. I’m very much a novice who is massively over-reaching their skills, but hopefully one of you knows how to help.

I’m trying to achieve something similar to the link below but I’m running into some problems:
https://www.snogproductions.com/projects/gq-jon-hamm

To account for the various image sizes is the sticky position centre-justified to the middle of the viewport? or have they used various image sizes in a single container class?

It seems more straightforward with standard images, but is there a way to do this where it’s populated via a collection? If so, will there be problems if some collections have more images than others?

Thanks for any light that can be shed on this.