Using logic similar to chapter six of https://www.scottaaronson.com/papers/philos.pdf , though, you can still draw a meaningful distinction between a normal JPG decoder, and your proposed JPG decoder that interprets the Declaration of Independence as illegal child porn. Your decoder must inevitably have a much higher information content in it, and is a "reduction doing all the work" in that case. So while you've got a mathematical point (no sarcasm, I understand what you are getting at), even on purely mathematical grounds I can argue a meaningful distinction; legally it'll be even easier.

Interestingly, it's a distinction that happens to more-or-less match our own intuitions about the situation, which is something interesting to ruminate about a bit.