As you all know, StackExchange's resident designer, Jin, has informed us that our site is due for graduation soon. Jin has been really awesome about getting input and ideas from the community about how the site should look. Now Jin is going to work on the design for the 404, Captcha, and Error pages, and he wants our input.
Usually, these pages have a little lighthearted cartoon or picture that fits with the concept of the page. As an example, these are the pages for English.SE's 404, Captcha, and Error pages. In those examples, the illustrations for each page are of the same style, this is not always the case, though. But on most Stack Exchange sites, these pages are lighhearted in tone: see the linked-to examples above, for example.
We've discussed some ideas in chat for these pages, but now we want to see what the greater community thinks. I'll post the ideas that have already come up in chat (I'll try to give credit where its due, but if I make a mistake, please let me know). Give us your best ideas, and vote up the ones that you like!
Edit: BTW, Captcha pages ("Are you human?") usually have a picture of some cute and/or funny non-human (robot or animal, etc.).
Further edit: A captcha page design has already been chosen and put in place (by Jin);
the others have not yet.
Update: these have all been implemented.