I recently mentioned the service called Sketchory, which contains 250K free-to-use sketches. Like the image on the right which I got when I asked for 'abstract'. Nice service. I have been involved with the problem of tagging images. In particular choosing among a number of image types. In Sketchory I noticed that it was difficult to get to useful sketches. They were asking for aid in helping tag. So I played along, tagging a few dozen images.
It's clear after doing this why it is hard to classify. People mostly sketch images of people and animals. Sometimes fanciful things, but rarely abstract ideas and even more rarely processes that I would like to have an image of. At the right and below is an image that was retrieved, of only about a half dozen, with the term 'writing'. Even people who are trying to do this kind of tagging usefully do not do it very well. I quickly started to use the simplest nouns to tag the sketches. These, like the tags that they have already are not very useful.
So there are 250K sketches, but it is hard to find the right ones.
It is still very hard to solve the automated general tagging problem. An AI problem. A number of attempts have been made at creating tags with 'Human Intelligence Tasks' (HITs), most notably with Amazon's Mechanical Turk. It is probably better to include some prefabricated classification capability that describes a number of useful classifications. Then have the person search among many examples. Google also has an image labeling game.
See Gene Smith's good book on the subject: Tagging.