His group Ontology2, has created the Ookaboo website, which contains digital images that they claim are either in the public domain and or under Creative Commons licensing terms. It appears to be a collection of harvested images from the web, in particular Wikimedia, where people place images that they wish to share with the world, so the free access is probably correct.
The interesting part to me, and I think to many of you, is how they are indexing the images , which is by means of the semantic web.
Here is their description of what that means.
Images on Ookaboo are indexed by terms from the semantic web, the web of linked data. Although you're free to find images through the human interface, automated systems can quickly find and use images through the semantic API.
Ookaboo has two goals: (i) to dramatically improve the state of the art in image search for both humans and machines, and (ii) to construct a knowledge base about the world that people live in that can be used to help information systems better understand us.
Semantic Web, Linked Data
In the semantic web, we replace the imprecise words that we use everyday with precise terms defined by URLs. This is linked data because it creates a universal shared vocabulary.
For an example, in conventional image search, a person might use the word "jaguar" to search for
• the animal
• the automobile brand
• the Jacksonville Jaguars (NFL team)
• the game console from Atari
• ... and nearly 30 other things that are listed in Wikipedia.
Note in the cases above, there are pages in Wikipedia about each of the topics above: it's reasonable, therefore, that we could use these URLs as a shared vocabulary for referring to these things. However, we get some benefits when we use URLs that are linked to machine-readable pages, such as http://dbpedia.org/resource/Jaguar, or http://rdf.freebase.com/rdf/biology.itis.180593
Pages on Ookaboo are marked up with RDFa, a standard that lets semantic web tools extract machine readable information from the same pages that people view.
Named entitiesThe above information is from their About us page, which I highly recommend you check out.
Ookaboo is oriented around named entities, particularly 'concrete' things such as places, people and creative works. With current technology, it's more practical to create a taxonomy of things like "Manhattan", "Isaac Asimov" and "The Catcher In the Rye" than it is to tackle topics like "eating", "digestion" and "love". We believe that a comprehensive exploration of named entities will open pathways to an understanding of other terms, and hope to extend Ookaboo's capabilities as technology advances.
Oh and yes their images are pretty good too, especially for those interested in buildings. and other "concrete" things.