On Area 51 I finally hit a captcha and was instantly depressed. It's the same as the Stack Overflow captcha :(
Can we please get something more specific and awesome for Area 51, hopefully falling in line with the rest of the theme.
If it's alien (well sort of) robots you want:
(assuming that this was the image I linked to in the first place)
There's always good old Cthulhu:
Not sure about ownership issues, though.
To make a kinder, gentler lord of the abyss and underworld, someone can just write (freehand):
I CAN HAZ CAPTCHA?
... on the image.
Alright, I think we should go with a Gaius Baltar administering the Cylon Detector Test theme.
Here he is administering the test, with benefits:
But that.... might not be fully work safe. So.... here's a pic of him looking at his own brain scan; it's "test-ish" so maybe that'll work.
We could go with just a Cylon:
Or Old Skool:
There have been any number of human-checking techniques in science fiction. It's an important plot element when the aliens or robots look like humans. Recent (relative to copyright) examples include V and Terminator. I can't think of an example right now that might have an image that's in the public domain.
I came across this movie-poster from Houdini's silent movie from 1918 called The Master Mystery which illustrates the human/robot relation quite well. Though I'm not 100% sure, I believe the robot could be an actual person in a suit..
So considering that possibility we should introduce a captcha to let robots proof that they aren't actually just disguised humans ;)
It should be an alien robot.