Thanks to everybody for the positive feedback on this. A few quick notes out of my experience with related tools.
Transparencies and smooth fades are neat but not functional for this particular purpose. As Simetrical pointed out abrupt changes are much easier to spot.
From one version to the next the changes might be very
few and very small. An abrupt change between two revisions of an image will maximize the likelihood for any change to be noticed, even if it's a single pixel.
Actual per-pixel subtraction gives interesting-looking results if the difference is large enough, but they can be sometimes difficult to relate to the original images and subtle changes will be difficult to spot due to their low contrast. Also, given a 0 to 255 range in value for each pixel, the result of a subtraction operation would be in the range -255 to +255. This then needs to be compressed back into a displayable 0 to 255 which effectively loses half of the information.
From an interaction point of view, it really should be a single
button operation: one click on the button and the lower layer comes forward, covering the previous layer. One more click and the layers are swapped again. There's no need for automated repeats of this: the user decides with a single click when to swap the visible image and how frequently. Generally this tends to be one or two clicks per seconds initially, to then slow down to one click every few seconds, while he/she focus on specific differences.
Hope it helps.
Ciao!
Manu