The library emphasized the effort that went into the production of their slides, which may have hurt their case—the effort expended was to make sure the slides reproduced the originals as exactly as possible (something of value to art scholars and historians) and thus were purposely devoid of originality.
From the poster above:
If the scanner does not add any originality, they are not entitled to claim copyright over someone else's work.
I think that is the crux of the issue, at what point does it not become the original. For example I have a photo with a lot of scratches, faded on one side and has corner torn off it. A better scanner itself can fix some of these problems, the person that has scanned the image can perform other operations such as reapplying the missing area (assuming it's not a huge portion of the image). IMO this is no longer the original, do you know of any case law in regards to that?