W3C - Image Annotation on the Semantic Web:

"The goals of this document are (i) to explain what the advantages are of using Semantic Web languages and technologies for the creation, storage, manipulation, interchange and processing of image metadata, and (ii) to provide guidelines for doing so. The document gives a number of use cases that illustrate ways to exploit Semantic Web technologies for image annotation, an overview of RDF and OWL vocabularies developed for this task and an overview of relevant tools."