Works great until it's double sided or more
It is just a simple thing, for simple jobs. Exploring an idea.
But with double sided PCBs, it would still be of value I think. Traces under components would be a problem, but it apart from that, it would allow a trace to be followed around the PCB from top to bottom through vias and pads.
If you find yourself tracing out a (simple-ish) PCB, even double sided, it's got to be a help.
Adding some cropping and centering would make it useful. You cannot rely on having two perfectly superimposed images.
If you have to open some more powerful image editor to adjust the images for your SW, you might as well finish the job there playing with transparency.
As to alignment, I envision just having a corner jig to position the PCB with stability, and a camera on a small tripod looking down on it.
Take a snap, flip the PCB, position into the corner, take another snap.
Use dumb old MS-paint to crop the images, using the corner jig edges as one reference, and just make sure that the X-Y pixels counts are similar on both images. Done.
Or just select the PCB area, also easy.
But yes, cropping, scaling, rotation and alignment facilities could be added.
I'm just presenting an idea here, not a finished, all singing, all dancing product.
This could be taken a lot further, with a facility to add overlaid component designators, marking pad and via positions, and maybe even component outlines.
Then a facility to click all the pads in a net, resulting in a netlist for input to some other EDA system.