Howdie,
As you probably know, administrators and other volunteers on commons
(and other wikis) wage a constant battle against people who upload
photos from web sites under bogus claims ("I took it myself!" etc.).
One thing that would be nice would be a tool to check whether and where
an image file is available somewhere else on the web.
I think we can get Google to help us in that matter.
Google Images, in order to build its thumbnail database, has to download
the files and compute thumbnails. They can, at the same time, compute a
hash of the file (SHA, MD5 or similar). Perhaps they already do.
If the hash is stored into the database, they can essentially answer our
problem. They already offer a SOAP programmatic interface (which does
not offer this feature); conceivably they could offer this "look for
identical files" feature, perhaps to selected partner sites.
Since they have offered us a hand in the past...
Regards,
DM