[3131s]

I was thinking along those lines, but don't you still make it easier by giving away some details about Waldo's facial expression, the relative size of his head, etc.? Still a pretty illustrative analogy though.

How about get the coordinates of Waldo's position in the image and hash them?

[WikipediasBad]

That would work except your friend would have to confirm you know only after finding Waldo himself and hashing it to compare the hash to your hash. The idea is that they don't have to know the secret, but you can demonstrate you know the secret.

[3131s]

Ah duh, in that case then I can't think of anything better :(