[DougMerritt]

It's not that you're utterly wrong; some transformations are irreversible, or close to. Multiplying each pixel's value by 0, assuming the result is exactly 0, is a particularly clear example.

But others are reversible because the information is not lost.

The details vary per transformation, and sometimes it depends on the transformation having been an imperfectly implemented one. Other times it's just that data is moved around and reduced by some reversible multiplicative factor. And so on.