From a pure photojournalist standpoint, and not reading the story or having any idea what the background is, look at this image, it shows a dog lying by a marked grave. You can think that it's sad or that the dog may belong to the person in the grave, but it is not a powerful image on it's own. Without the story to give it meaning it is a simple image. I'm sure that the circumstances surrounding the flood, the mass grave and the tragedy that goes along with it give the reader that sence of what is heartbreaking, sad, etc. Once you know the meaning behind the photo, it makes you think, which means the photographer has done his job.
Stepping back to photos that were powerful, the girl running naked towards the photographer after napalm burned her clothes off in Vietnam, if you are too young to remember it, of have never seen it, that single image is a powerful image. Or the photo by Eddie Adams of the police officer executing the Vietcong prisoner in the street, those are powerful images, that during the time needed no words to give them any impact.