[mikestew]

all they would have to do is change a single pixel and/or re-export as a different format

This sounds plausible if you don't know how perceptual hashing works:

https://en.wikipedia.org/wiki/Perceptual_hashing

[mrguyorama]

Your own links talks about how perceptual hashing hasn't been proven to be robust enough for this use case, and also introduces a new problem: Hash collisions, such that you can generate images that hash to the same perceptual hash as an illicit image.

[mikestew]

Robust or not, it's a little more nuanced than a file hash that goes out the window because a single pixel changed, as implied by GP.

[_manifold]

Yes, I did some more research on this as well. Apparently perceptual hashing is also susceptible to some pretty simple attacks:

https://arxiv.org/pdf/2111.06628.pdf