After months of experimentation, Google choose to show text ads above image search results. It's not clear if the ads are actually useful for those who visit Google Image Search to find images, but Google mentioned last year that the volume of commercial queries has increased significantly.
"Whenever we make changes like these, we carefully evaluate users' reactions to ensure we're holding true to our basic principles: that ads by Google should always be relevant and useful. Of course, these experiments benefit Google because they generate revenue from new sources — but by ensuring that we show the right ads at the right time to the right people, we'll add value for users too," explained a Google blog post from November.
In 2006, Google estimated that it could earn $80-200 million a year by including text ads in Image Search. Google decided not to monetize Image Search because the experiments showed that people used the service less. I wonder if the experiments from 2008 showed different results.
It's worth mentioning that the only other important image search engine that shows ads is Ask.com Image Search.