I TAG Image Annotation System Using Visual and Textual features

Also Google has made attempts towards this by involving the user in understanding the semantics of the images by developing an Image-Tag game Image Labeler [11].

http://www.cs.ucla.edu/~cardenas/papers/VLC10-2-Barai-Cardenas%20Image%20Annotation%208%208%202010%20best%20resolution.pdf