Overlay printed maps on digital maps

As a user, I want to take a scanned old paper map (or a scanned aerial photograph), and overlay it on top of a modern digital map.

When I'm viewing the page, I want to be able to adjust the transparency of the overlaid scan, so I can see the modern map beneath it.

This is very helpful when trying to identify the current location of historical features.

Here's a paper map overlaid on a digital map on a Drupal site, though it had to be set up by the administrator and doesn't have the option to adjust transparency: http://gwulo.com/1924-map-hong-kong-western