The frog, in reality, just jumps out of the pot when the water gets too hot.
Empirically, slippery slopes have always been an extremely common way change is driven in mass social thinking. This has been formalized as the "Overton window": https://en.wikipedia.org/wiki/Overton_window
Saying, "We're still three steps from my issue but this thing that, in and of itself I have no problem with, can't happen because it might lead to the thing I have a problem with." is not a valid argument.
To try and bring it back into focus, it's a real problem that this "one pic" isn't being shown. It's not a slippery slope conversation, because we're already at the thing I/we have a problem with. No need to argue the "this paves the way for worse things" because this is the worse thing! We're here!