Is the image below a puzzle? It's to hard for me in that case!
If it is not; I think it should be. And I think it should be a community contest to create a visually as interesting piece which is at the same time a hidden puzzle. I like the image, it's quite intriguing, it even got a key. But it's aesthetic appeal makes me think it just there for show and not an actual puzzle.
No, the 404 page isn't a puzzle.
It's just part of the overall Puzzling SE site design. We have a very similar thing on the T-shirts we were sent when the site graduated.
As for creating a visually interesting piece which is also a puzzle, that does sound like a nice idea for a community contest on the main site. Perhaps you could propose something along these lines for a topic challenge? You'll have stiff competition though - there are already some truly excellent visual puzzles on this site, from creative geniuses such as Alconja, Avigrail, and Bmyguest.
