Imagine yourself minding your own business when a tourist at Times Square snaps a picture with you walking in the background and posts it on a public site. Using a search engine like Polar Rose, your boss could easily find out you were out and about on a day you had called in sick.

Police, stalkers and spouses also could use the technology to track where people have been ¡ª for example, if someone has attended anti-war protests in multiple cities.

"I don't think we have all the answers quite yet," Nyholm said, adding that people went though similar debates years ago when search engines began indexing text.

"A lot of pictures have been published, and privacy has been assumed due to obscurity," he said. "This will highlight the fact that there is no such thing as privacy by obscurity."

It's not clear how well the service will work. Facial-recognition technology isn't error-free ¡ª people get tans; some occasionally wear sunglasses. And the human component will help only if a large number of people participate; many other human-assisted search engines have produced lackluster results.

Simon Davies, director of Privacy International, said that regardless of the service's effectiveness, technologies such as Polar Rose underscore the need for a global debate on whether to place limits on what search engines can index and to give individuals greater say.

Without such dialogue, he said, "these technologies will keep drilling into information to create search dimensions which are infinitely more powerful than we could ever imagine."

And he rejects Nyholm's contention that just because an image is accessible, it's fair game.

Whenever information becomes easier to find and access, "a whole raft of new privacy issues are always created," he said. "When people place their photographs on the Internet, they do not expect them to be searchable."

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