That certainly is a challenge. A bigger portion of the challenge is the fact that randomization is (I believe) by date, and not by visit. Meaning, even if you set the loaded images to randomly pull in from a collection, the set of 5 will be the same for everyone who visits on a given day, no matter how many times they visit.
That could have been changed since I last tried it (a year or more ago), but I doubt it.
The client has to realize that visitors are very unlikely to hit the site the 16 times it would take to see all 80 superheug images given a perfect randomization coincidence, and even if they did it would likely be because they are coming to the site for a specific reason and would be increasingly unlikely to sit and stare long enough to even see the second image in the sequence…
If it were me (and it has been me), I’d suggest a masonry layout where the images are displayed as part of a large block of images. Each one being something like 20vw wide x 15vw high in 4 rows of 5. That gets you 20 at a time and it could be used as a mid-page accent throughout the site. You could pretty easily set those to random and as a person visits different pages of the site, they’ll be exposed to all of the different images, even if not in the detail the client is envisioning. Then just use the top images as a hero on each page, with the availability for a person to see each of them in their full-resolution glory if they CHOOSE.