It pollutes the data pool. The rule of gigo (garbage in garbage out) is used to garbage the AI results.
Basically, it puts some imperceptible stuff in the image file's data (somebody else should explain how because I don't know) so that what the AI sees and the human looking at the picture sees are rather different. So you try and train it to draw a photorealistic car and instead it creates a lumpy weird face or something. Then the AI uses that defective nonsense to learn what "photorealistic car" means and reproduce it - badly.
If you feed a bunch of this trash into an AI and tell it that this is how to paint like, say, Rembrandt, and then somebody uses it to try to paint a picture like Rembrandt, they'll end up getting something that looks like it was scrawled by a 10-year-old, or the dogs playing poker went through a teleporter malfunction, or whatever nonsense data was fed into the AI instead.
If you tell an AI that 2+2=🥔, that pi=9, or that the speed of light is Kevin, then nobody can use that AI to do math.
If you trained Chat GPT to explain history by feeding it descriptions of games of Civ6 them nobody could use it to cheat on their history term paper. The AI would go on about how Gandhi attacked Mansa Musa in 1686 with all out nuclear war. It's the same thing here, but with pictures.