John Locker said he stumbled across the images, which were beamed over an insecure satellite link and were not encrypted, in November.
Locker said the freely available pictures by both manned spy planes and drones can pinpoint a location to within six feet.
"It's frightening -- I am amazed," Locker said. "Even before September 11, this is not the sort of stuff that should be shown openly."
Locker said he tried to warn NATO that the pictures could be viewed by anyone with basic equipment.
"They eventually told me it was a hardware constraint, they were aware of it and they thanked me for my concern," he said.
NATO said it was not concerned about any possible security breaches but American officials said plans were in hand to encrypt the data.
Locker, who picked up the broadcast from the Telstar satellite over Brazil at home on his satellite dish, stressed he was not tapping into anything.
"This is not an intercept," he said. "I am not a hacker -- this is free to air programming."
"I would question if this could put troops at risk on the ground. Those pictures are within real time of three seconds," he said. "It is just stunning."
He said pictures he has seen covered military exercises on the ground in Macedonia and further north in the Sarajevo area in Bosnia.
Clearly visible were troops on the ground, armored personnel carriers and a helicopter whizzing underneath the camera.
Viewers tuning into the satellite this week were reported to have been able to watch a security alert around the U.S. Army's headquarters at Camp Bondsteel in Kosovo.
Last week, the spy plane provided airborne surveillance for a heavily protected patrol on the Macedonian-Kosovo border near Skopje.
Locker said: "What I suspect is that they are using military satellite capacity for Afghanistan as their top priority. As that capacity runs out, they may be using a commercial satellite as a backup."
Locker is a freelance journalist who writes for satellite communications magazines.
"We can see dozens of satellites in the sky," he said. "This just happened to pop up on one of the satellites last November. It appeared to me to be of military origin."
Clips from the feed have been transmitted by the International Consortium of Investigative Journalists on their website, www.icij.org.
Richard Perle, chairman of the Pentagon's Defense Policy Board, told the BBC plans were now in hand to encrypt the data.
"We have discovered in the period since September 11 how important this sort of real-time intelligence is," he said. "Now we are making much better use of this kind of information and it will make sense to encrypt it in the future."
Major Bill Bigelow, a spokesman for the U.S. European Command in Germany, said the images did not constitute intelligence.
"Raw information such as that video does not mean intelligence," he said. "Intelligence means analysis of data that comes from many different sources."