"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.