Generative art is interesting in itself, so plenty of nice images can be encoded. I've checked JPEG-XL Discord channel and there's a small group of people creating these images for fun.

Code Golfing challenge:

- Write an efficient Bitcoin miner that runs in a JPEG.

- Steal the memory of a user using the Spectre vulnerability.

Generate an image which is itself a fully functional program writen in the Piet esolang[1].

[1] https://www.dangermouse.net/esoteric/piet.html