Stack overflow's custom 404 image is a polyglot that prints "404". Our 404 image is just 404 in a blue color. Some theme-graduated sites have a gray 404. Stack overflow users requested a custom 404 image, like Physics.se or InfoSec.se. Some sites like crypto.se have their own custom image. We should too.
The design of 404 page is a part of site's custom design, something that PPCG does not have. It is not something that a site requests.
It's true that sometimes a custom 404 page is just the number 404. It's still a custom design: https://rpg.stackexchange.com/404 uses a font matching the site's logo, while https://money.stackexchange.com/404 looks like a number on a banknote.
The Information Security site is an unusual case: initially it used a frame from the movie Beautiful Mind on its 404 page, but when the copyright implication was realized, the image was removed.
Whatever 404 page will be, it has to match the overall site design, so it's pointless to discuss it separately from that. Obtaining it ahead of the design isn't an option.