It did not take long, after the attacks at the finish line of the Boston marathon, for images of the carnage to start rocketing around the web. The stills and the video footage streamed across our screens even before the police or the local government could confirm what had happened. Every event is a font of images now, every bystander a photographer.

But one photograph in particular, circulating endlessly on social networks, has stood out—a professionally shot one, not a camera-phone snap. It was taken by Charles Krupa of the Associated Press, and it shows a man in a wheelchair being rushed to medical attention, his body mutilated by the blast. He is, to appearances, a spectator and not a runner. His right leg is only partially visible, although his right knee seems badly damaged. He grips his left thigh, wrapped in a tourniquet. But below the thigh his left leg is mangled. His foot is entirely gone. Skin dangles in large, red strips. His left tibia, covered in dark blood, protrudes in defiance of bodily integrity. Ligaments or tendons twist in the air.

Several news organizations have blurred the victim’s face – but online, of course, the unaltered version still circulates. I have looked. His expression is one of shock rather than agony. It’s a dreadful image, and as news came in of multiple leg amputations among runners taken to Massachusetts General Hospital, the photograph for me became an awful reminder of all the maimed athletes who had just been running across the city, with bodies still intact.

In the face of such violence, however, some people’s instinctive response has been to dismiss Krupa’s photograph as too ghastly to be shown. Several news organizations have cropped it to eliminate the mutilated leg. At least one online outfit superimposed a “warning: very graphic” disclaimer over the image, shielding it from all but the most strong-stomached viewers. The media always twist themselves into knots debating the ethics of publishing such images. But viewers aren’t the media. We have a different set of obligations, and in a moment when such images are instantly accessible to all, there is really no excuse not to look. As Susan Sontag maintained (haughtily but not incorrectly) in Regarding the Pain of Others, her 2003 book on war photography, “someone who is perennially surprised that depravity exists … has not reached moral or psychological adulthood. No one after a certain age has the right … to this degree of ignorance, or amnesia.” It is supposed to be difficult to look at images of bodies in pain. We are supposed to recoil at first. Images can’t possibly convey the full reality of suffering, but nobody is asking them to. What they do convey, better and more efficiently than words, is an imprint or a reminder of human cruelty and human pain.

As intimations of terrorism—a tactic dependent on the circulation of images—floated through the media yesterday, the image of the man with the blasted leg reminded me of something else: of the overwhelming cache of war photographs from Iraq (and Afghanistan, and elsewhere) that American media have largely declined to show. They have circulated freely on the Internet for years, and anyone with a computer can see the corpses of Iraqi civilians with half of their faces blown off, or Iraqi children whose tiny bodies have been consumed by fire. Our world today is not only supersaturated with images, but with image producers. The wars of this past decade have been the most photographed in human history. Yet the actual influence of these images of war has been surprisingly low; in America, at least, most people still haven’t seen them.