NEW YORK — A Swedish start-up is combining software and humans to help make photos and other images more easily searchable online, raising privacy concerns as the technology eases the tracking of people across Web sites.

Traditionally, search engines analyze text surrounding an image on a Web site. So a search for "Bill Gates" might produce a photograph captioned with the name of the Microsoft Corp. chairman. But a search for a reporter's name might produce that same photograph if it had accompanied an article he had written.

Polar Rose AB is bringing facial-recognition technology to the mix. Its software scans everyday images for about 90 different attributes. If the software finds a match with images in a database, it concludes the two photos are of the same person.

The company, among many start-ups seeking to improve image search, believes its technology is noteworthy because it creates 3-D renditions of faces in images, allowing the computer to account for slight variations in angles and lighting.

Nikolaj Nyholm, the company's chief executive, said testing has shown up to 95 percent reliability with sets of 10,000 photos. But he said that as the collection grows — there are millions, perhaps billions, of photographs on the Internet — reliability diminishes because, well, many people simply look alike.

That's where humans come in. Early this year, the company will distribute free plug-ins for Microsoft's Internet Explorer and Mozilla's Firefox browsers. People who post or view photos could add information such as names; there might be the occasional error, but enough people filling in the correct answer would make that rise to the top.

The idea is to label every face, even ones in the background, whether posted on a Web journal, a photo-sharing site like Yahoo Inc.'s Flickr or a social-networking hangout like News Corp.'s MySpace. The service won't index images on personal computers or password-protected sites.

Polar Rose plans to sell ads and premium services but won't charge for the basic use of its plug-ins or search engine, which is still in a "beta" test phase.

But there's still a cost: privacy.

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 — for example, if someone has attended anti-war protests in multiple cities.