Robert, I did the same exercise myself and found huge distortions in the images. Basically, I defined a triangle with the big crater, the small "twin" craters off to centre left, and a small crater at top centre. But I found that I couldn'r match any two images without huge distortions.

Did you find this too? I didn't notice the same with Spirit. Maybe the camera was pointed off sideways and there is a trapezoidal distortion of the frame?

Oh, and you said "...but it was shifted enough from the other two that the Sun was probably at a much different angle." - Ithink you'll find that the Sun was at the same angle to Mars, just that the spacecraft was at a different position/angle. The Sun don't move much in the time it takes a parachute to drop 500m in a 6 mbar atmosphere! :-)