What does "many" refer to? "Many images"? I also don't understand the logic of "only when."
Context:
As with any propaganda or PR, the images are carefully staged. And
many make foreign news headlines only when experts and photo editors discover that North Korea is digitally altering them. For instance, in a picture distributed recently by state media, troops and hovercraft land on a barren, snow-dappled beach. Experts say some of the multiple hovercraft have been copied and pasted into the image.
More:
http://www.usatoday.com/story/news/world/2013/03/30/kim-soft-side-propaganda/2037859/