Many fast-growing parts of the Web, such as Facebook or Google's Picasa, are populated with user-generated images. They could become a rich advertising opportunity with the right technology, says Yang. "Many photos in online photo albums and video scenes do not have texts to describe them," he says. "People browsing through their own or others' online photo albums are a potential audience for adverts." Today, he says, it's impossible to reach people where there is no surrounding text.

To match an advertisement to an image, the group's software first converts the image to a collection of words. The software was trained to do this by crawling around 60,000 images on Flickr that have tags added by users.