How would you illustrate that an image is encrypted? I want to show the original image and then the "encrypted" one. I want people to see that it's based on the original photo but I don't want it to look like the encryption is too insecure. This is a bit tricky, since it always is insecure if you can recognize that it is based on the original image but using "real" encryption would make the illustration less effective.
If you're trying to communicate the message that the encrypted image is unrecognisable as the original image, with a 'before' and 'after' pair, then at least part of the 'after' image has to be unrecognisable as the original.
Assuming it has to be both unrecognizable, and immediately recognisable, that sounds impossible... but one way to do it would be showing the transition from one state to the other.
Here's a really crude example. You'll know best how to make the general principle fit what you're trying to do.
Another option would be have one complete state at one end, one at the other, and one or more intermediary states between them, so it's clear that that state 1 becomes state 2, without compromising state 1 or state 2.
Or maybe 3 images: original [arrow->] 100% encrypted [arrow->] original, and so it's the flow of the diagram that says that the encrypted image is still the original.
I think the only answer, given your constraints, is to obscure the image as far as can be without making it totally unrecognisable.
I would suggest applying a colour noise filter over the top and then reducing its opacity until the image can just be discerned.