Solving Instagram's Unshredder with Mechanical Turk and $0.50
Another possible way to do this is present the Turker with two  vertical pieces already placed together. Ask them a yes/no question: do these two images match?<p>Iterate intelligently over your favorite sorting algorithm until you've placed all the images!<p>Note: requires more Turkers but each answer could be worth much less, maybe around $0.01