You show a jury a picture of the minor and a reproduction of the image in question. It may not matter too much if the creator intended the image to resemble the actual minor; only that they intended to create a sexually explicit image of a minor, which happens to resemble an actual minor. The marginal cases won't be prosecuted.

But some (a lot of) idiots will do your hard work for you. They'll share torrents of "<insert underage celebrity> Nude.mpv" that has been AI-generated, or post an erotic drawing of their favorite streamer's young daughter to a chan board, labelling it as such. The law allows the prosecution to nail those cases to the wall.