CompSci wizardry time: file hashes can be vastly altered by changing a single pixel, and not even perceptibly, because hashes are extremely random by design. The point of a hash is to be vastly wildly different for any given input so you can see if things are even slightly changed, so a single character in a text file or a single bit flipping in binary will throw it off.
Perceptual hashing, which hashes the image in itself, instead of the binary representation of it (the data 1s and 0s a computer sees), returns similar hashes for similar images. This is a bit harder to fake than "I changed the RGB of a single pixel by 1", but is also defeatable. HSV noise, putting random shit (emojis and stickers, even) on the image, rotating enough, left/right flipping, stretching, tiling, etc, defeat this too, but it takes more effort than clicking a single pixel on a smartphone. On the other hand, this does not seem to be as widely used and may incur costs to hosts.
I remember asking Null about this in particular and he said the fees were pretty fucking stupid, for what really should be a service to humanity.