A picture, they used to say, is worth a thousand words. Now it’s more like half a million dollars. That’s what Us Magazine is said to have paid last week for some innocuous photos of Brad Pitt and Angelina Jolie, the Hollywood “couple” of the moment, strolling along a secluded beach in Mombasa. The pictures set off a fierce bidding war between Us and its weekly rivals People, In Touch, and Star.

But the market for such photographs is no longer limited to the celebrity weeklies. Magazines such as In Style and Life&Style, along with many women’s magazines, fill their pages with pictures of celebrities, dressed in designer fashion at “red carpet” events like award shows and openings.

Nor is the appetite for such pictures limited to the United States.

“We can now sell pictures of these people all over the world,” said Kevin Smith of Splash News. “It’s amazing. There is really a worldwide fascination. Even in places like Estonia and Croatia.” Splash News scored last week’s second biggest scoop, the first photos of Tom Cruise and Katie Holmes – 16 years his junior – being unabashedly affectionate, out on the town together in Rome. Both stars, by the way, just happen to have movies coming out next month, and some are already declaring that their relationship is based more on publicity than passion.

Splash News, Bauer-Griffin, X17 – all headquartered in Los Angeles – along with Big Picture, the London based agency that scored with the Brad Pitt photos, are currently the hottest of the paparazzi agencies. Many of their photographers are British and Australian, according to J.P. Pappis, the highly respected head of Polaris, a New York-based agency that deals with news and features. “They have a newly aggressive style,” Mr. Pappis said. “Once upon a time, a photographer from the Associated Press or Reuters could shoot a news story in the morning and then cover a movie opening at night. That doesn’t happen anymore.”

The real challenge is catching celebrities when they are not camera-ready but are caught off-guard: shopping, eating, playing with their children, even taking out the garbage. Best of all is to catch them, if possible, when a new romance is budding or when they are with someone they shouldn’t be with.

“The situation has gotten completely out of hand in Los Angeles,” said Peter Howe, a former New York Times Magazine photo editor, and the author of the soon to be released book “Paparazzi and Our Obsession With Celebrity” which includes interviews with photographers about some of the biggest celebrity news incidents in recent time. “The money is getting bigger and bigger,” he said. “I don’t doubt there will be a million-dollar set of photos in the future.”

Yet as the money gets bigger, the situation may also be getting more dangerous.

“Right now there are so many paparazzi chasing after the same few celebs that many people fear that there could be another Princess Diana-like tragedy in L.A.,” Mr. Howe said.

Kevin Mazur, one of owners of WireImage, another successful agency, sees the same problem. “I think it is very sad,” he said. “I feel sorry for Britney Spears. She’s pregnant and every time she goes out for a drive she is being chased by at least 20 guys in cars. It’s dangerous for her and for other people. Personally I don’t call them ‘paparazzis.’ I call them ‘stalkerazzis.’ If they didn’t have cameras they would be locked up for being stalkers.” Mr. Mazur, unlike some of his competitors, has close relations with many stars and their publicists.

Currently Ms. Spears doing practically anything is the most wanted. She’s followed in appeal by Brad Pitt, Mr. Pitt’s estranged wife Jennifer Aniston, and his good friend and possible girlfriend, Ms. Jolie. But hot celebs can grow cold very fast. At the start of their May-December romance, Harrison Ford and Calista Flockhart were chased by the pack. Now they are practically ignored.

“The stars of reality television shows during their 15 minutes of fame are also very interesting to paparazzi because by taking their pictures they can make a lot of money fast,” said Mr. Howe. “And unlike stars they don’t have the same kind of protection. Magazines want their photos, too. You know the biggest seller issue for Us magazine had ‘Bachelor’ Bob on the cover – not Nicole Kidman.”

Realizing the value of their image, some celebrities now sell their own pictures. Some invite a particular magazine in for an exclusive photo session and give the money they are paid to charity. That’s what Julia Roberts, a perennially “hot” star, did in February when she let People take pictures of herself and her 12-week-old twins, Hazel and Phinnaeus. More recently Star Jones, co-host of “The View,” made a deal with InStyle to cover her wedding. When she discovered that her fellow cast member, Joy Behar, was also taking photos, she gave her a tongue-lashing. Their relationship has been contentious on-air ever since.

Magazines in Great Britain like Hello! and OK! have always paid celebrities for access to their homes and on special occasions. Catherine Zeta-Jones and Michael Douglas sued Hello! magazine for publishing “unauthorized” pictures of their wedding because they had sold what they said were the “official” pictures to OK! And they won their case.

Occasionally, what looks like a casual caught-on-the-sly paparazzi picture is, in reality, a carefully staged moment organized by a publicist to get the greatest possible attention for his client. “It is not the invasion of privacy that really bothers celebrities,” Mr. Howe said. “It is the lack of control. I am not saying that paparazzi are the nicest guys in the world or you’d want them for your best friend, but there is a lot of hypocrisy here. For example, there is a famous picture of Gwyneth Paltrow and her husband, Chris Martin, leaving the doctor’s office after they found out she was pregnant. He is kissing her stomach. It looks like a very intimate spontaneous moment, but it was a setup.”

When tracking the most wanted, least cooperative celebs, photographers are frequently as relentless as bounty hunters, employing James Bond style technology. Phil Ramey, a veteran master of the game – he took the “unauthorized ” photos of the Zeta-Jones/Douglas wedding – once rented a submarine in order to shoot photos of Princess Diana on an isolated Caribbean island. Many of his peers combine computer technology and tracking devices with top grade intelligence provided by dozens of highly remunerated tipsters. They have networks of parking valets, waiters, concierges, maids, and travel agents.

“If these guys were covering regular news, they would be phenomenal,” said Mr. Howe. “They are so relentless. They get so much information and know how to use it. They would have solved Watergate in a day and a half. And they would have known, absolutely for sure that there were no weapons of mass destruction.”

Still, even the canniest paparazzo can meet his match, and he has increasing competition from the many ordinary people who carry cameras all the time. The photographs of Britney Spears’s first Las Vegas wedding, snapped up for a hefty sum, were taken by an amateur who just happened to be in the right place at the right time.