In the imaginary dream world that Adobe, Google et al live in, the final file does indeed have a signature.

Each piece of software in the chain must use TPM-like technologies (yes, even GIMP) to make sure it's running a "legitimate" build of the software, on "legitimate" hardware, and re-sign the file at each step along the way (using keys provisioned during some flavour of remote attestation flow, or using a RA-authenticated remote-signing oracle).

The final file embeds every preceding manifest, so you can "verify" all the way back to the original.

If this all sounds patently unworkable, that's because it is.

OK, but given that GIMP is a general purpose tool, what use is the signature if all of them verify it, when I can drastically change the image to whatever I want it to be?

The manifests at each step can embed a thumbnail (although this is optional, iiuc!), so looking at the thumbnail history it should be obvious that the edit was significant.

Yeah - I totally would not support that!

I can't wait for poking memory of applications running on my computer to become a felony.

It already is.

No it isn't, cheatengine exists, Q.E.D.

If you use cheat engine to inject child porn into an image viewer is it legal?