Many of the people might know that search engine displays the image result by means of "alt" tag specified in <img> tag. But, google also used the below specified techiques to bring out the qualified search results.
http://images.google.com/imagelabeler/
In this link, you and your partner might shown an image in the same time. So whatever you think of while seeing this image you can type it, if your thought and your partner thought matches then you get the points based upon the category. This is how the Google filters the images search result. Because if most of the people type the same word then it might be true. As the search engine was not able to see the image it used these types of techniques to give better search.