So photons just started doing this in recent years?

They have always done that.

The less complicated the lens, the less it controls for flaring, aberrations, loss-of-sharpness etc etc.

Why are modern prime lens so big and heavy?

Simple (not really): Every time a photon passes through a medium, like say - glass - that photon gets distorted, at least to some extent. You must now correct for that distortion and you do so by… wait for it… passing it through another medium.

Uh oh. You’ve distorted it in an entirely new and exciting way. Which must now be corrected for. Guess how that’s done?

It’s enough to make an engineer weep.

Read this: https://www.dpreview.com/opinion/9236543269/why-are-modern-50mm-lenses-so-damned-complicated

Pixabay’s usage restrictions are virtually identical to everyone else’s including iStock.

" You cannot use any Content on or in conjunction with anything pornographic, obscene, offensive, illegal, immoral, infringing, defamatory, hateful, threatening or libellous in nature "

The clause is there specifically to keep the images from showing up on adult web sites, which is exactly the use the OP wants (even if the site is fake and made-up just for the film)

There is no copyright on AI images

Nitpick: Be sure to check that first as some of the online generators absolutely do restrict usage of the content you create using their tools.

So my question is are there any stock image websites that would allow such use of their images? Or is my best bet trying to find a photographer who takes such images and making a deal with them?

Neither.

If this is just going to flash by with little time for critical examination of each image then why not just generate the content yourself? There are plenty available at the end of a google search. They produce results good enough to get thousands of thirsty followers on instagram so they’d be more than good enough to use in a film where viewers aren’t even being given the chance to really look at them before they’re gone.

Double check the usage restrictions (if any) of generated content and just make whatever you want.

No licensing issues, no model records to maintain etc etc.