Dear all,
thanks for all your help with answering questions and giving feedback over the last couple of months. I'm happy to say that we're finally at a stage where we've hashed 22,452,638 images from Wikimedia Commons and launched Elog.io in public beta: http://elog.io/
Elog.io is an open API as well as browser plugins, that can query and get information about images using a perceptual hash that's easy and quick to calculate in a browser.
What the browser extensions allow you to do is match an image you find "in the wild" against Wikimedia Commons. If it can be matched against an image from Commons, it'll show you the title, author, and license, and give you links back to Wikimedia, the license, and a quick and handy "Copy as HTML" to copy the image and attribution as a HTML snippet for pasting into Word, LibreOffice, Wordpress, etc.
Our API provides lookup functions to find information using a URL (the Commons' page name URL) or using the perceptual hash. You get information back as JSON in W3C Media Annotations format. of course, the information you get back is no better than the one provided by the Commons API, so if you already have a page name URL, you may as well query it directly, and rely on our API only for searching by perceptual hashes.
The algorithm we use for calculating perceptual hashes, which you'll need to query our API, is at http://blockhash.io/
Sincerely, Jonas