Eh. It was some kind of hash of the image. I was not involved in that project, so can't tell you exactly how it worked, but the images were "signed," and someone figured out how to "re-sign" an altered image.
Sure but conceptually no one should've been able to crack any hashing scheme anyone half-way decent at their job could come up. SHA256 is the default and it's unbroken. Even SHA1 has scant few known collisions. So like...what the heck were they hashing and how that anyone was able to crack it?
I think it was a fairly well-known technique.