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