Speaking Image, a web tool for annotating images, is available in beta. From their website:
Speaking Image is a web application which allows users to upload high-resolution images, work on them adding information over specific areas of the image and interact with other users through standard social network tools.
Videos describing how to use Speaking Image are available on their help page. The annotated version of Pablo Picasso’s Guernica shows some possibilities for annotating works of art with Speaking Image.