Against a gray expanse of pavement, a man in a neon orange jersey sprawls on his back, his gray hair wild from the force of the explosion that has knocked him to the ground. His gaze is turned toward three police officers in yellow vests who have unholstered their guns and walkie-talkies and who sprint through smoke and haze.

By now you have seen this photograph a dozen times, one of hundreds of visual artifacts for a tragedy that played out in a horrifyingly visual manner — on screens, through footage, in pictures.The Boston Globe posted it last Monday afternoon, and by that evening it had been retweeted 2,300 times. The runner was identified: Bill Iffrig, 78, a grandfather and retired mason from Lake Stevens, Wash., who later told CNN how the shock waves had made his legs “jitter.”

His identity almost didn’t matter. He was simply the Fallen Runner.

A week after the Boston Marathon bombing, we are collectively composing the first draft of history. We curate the images that will come to represent this week: A pair of grainy stills from a department store surveillance camera. Newsreel of police officers, swarming a Boston suburb in the middle of the night. Newsreel of police officers, cheered by the city. Endless online galleries prefaced with, “Warning: Graphic.”

We do this again and again — during wars, after assassinations. Chaos is organized into pixels, frenzy is made still and two­dimensional. We catalogue and remember.

The Sept. 11 office worker, coated from head to toe in a blanket of yellow dust. The Oklahoma City firefighter, cradling the unmoving body of a child amid the rubble of the Alfred P. Murrah Federal Building. Napalm girl. Afghan girl. The monk seated in peace, burning alive. The ordinary people we know by face rather than by name, and who we remember only because the world went pear-shaped, and they happened to be caught on film.

Distilling the moment

The day after the marathon, the photograph of Iffrig appeared, large and high-definition, on the front page of dozens of newspapers around the country: the Chicago Tribune, the Arizona Republic, the Orlando Sentinel, The Washington Post. By Wednesday it had gone international. In Ireland, Germany and Belgium, Iffrig lay on the pavement under headlines in foreign languages. Sports Illustrated announced that the picture would become its cover for the week of April 23.

“It’s the simplicity,” says Sarah Leen, a senior photo editor at National Geographic Magazine, when asked about the Iffrig photograph. “It’s full of symbols that represent the event. . . . You have a runner, so you know it’s a race. But the runner is in distress, so you know something bad has happened.”

It is, emotions aside, a well-composed photograph. The colors, the lighting and the mood all interact as if they had been organized, rather than desperately captured by Boston Globe photographer John Tlumacki, who himself felt the force of the explosion.

The Iffrig photograph, Tlumacki says in a telephone interview, conveys the sounds and the smells — “the firecracker smell” — that were present at the finish line that day. It’s the photograph he took that he feels represents what it was like to be there.

In anniversary remembrances, we are likely to return not to images of alleged bombers Dzhokhar and Tamerlan Tsarnaev, but rather to the victims and heroes: to the man in the cowboy hat, running alongside the man whose legs had been blown off. To exhausted marathoners-turned-EMTs. To that initial moment when the earth was displaced and the ground shook underfoot.

Historically, the photographs we tend to remember are not the ones that capture the whole of a tragedy — a broad battlefield — but the ones that depict the personal effects of one: a naked girl running down a dirt road, her skin burned from a South Vietnamese napalm attack. The ones that evoke the specific emotions of a specific time.

“Human beings are empathetic creatures. We deal most effectively one to one,” says Ann Shumard, the curator of photographs for the Smithsonian’s National Portrait Gallery. “It’s hard to respond to a mass of people.”

A single image of a single person “can be tremendously evocative and distill the essence of a tragedy,” Shumard says. “To focus on just one person in the midst of all this swirling chaos — I think that’s probably the first step to coming to terms with what has happened.”

The image of the fallen runner reminds Shumard of other images. “The young woman kneeling over the body at Kent State. That image is just so riveting and heartbreaking — the disbelief and despair.”

‘Involved in something big’

The young woman at Kent State was Mary Ann Vecchio. She was a 14-year-old runaway from Florida when she joined some new friends for a Vietnam war protest that left four unarmed college students dead on May 4, 1970. An image of Vecchio kneeling over the body of Jeffrey Miller won the Pulitzer Prize that year. In the black-and-white photograph, her left arm is outstretched, her right palm is upturned in a gesture of helplessness, and her mouth is an anguished, open wail.

“Every time I see that image . . . it’s like an injury that reinjures itself,” Vecchio says. “It gets in the marrow of your bones.”

Today, Vecchio is 57, a respiratory and massage therapist back in Florida. She works for the Department of Veterans Affairs. She comforts Vietnam veterans. She will always be the girl in the picture.

Once a month, sometimes more, she gets letters. “School kids doing projects. They ask me my opinion on events, what I thought about the war. The Newseum has asked me for my autograph.”

She is not a celebrity, not in the traditional sense. But she is one of those ordinary people snatched up into extraordinary circumstances, a totem for her time.

Time marches on, and we look at photographs, yellowing around the edges, piecing together a scrapbook of history. Slide Bill Iffrig in next to Mary-Ann Vecchio. Or maybe next to Juan Romero, whose deep, rich voice is still slightly accented, despite living in the United States for more than 50 years.

Romero was the 17-year-old busboy greeting Bobby Kennedy in the hotel receiving line just as the senator was shot and killed. That’s him in his white kitchen uniform in the photographs, kneeling over Kennedy’s crumpled body.

After the senator died, Romero was taken to the police station and questioned as a witness. He was released the next morning and, not knowing what else to do with himself, immediately got on the bus to go to school. “I was looking at my hands, and I still had blood on my fingernails,” Romero remembers. A woman reading a newspaper tapped him on the shoulder. “This is you, isn’t it?” she asked. Seeing the photograph “is when it started to sink in that it really did happen. It wasn’t a dream. It wasn’t an illusion. That’s when it really hit that I was involved in something big.”

For years, it affected his entire life. “I couldn’t do my job,” says Romero, now 62, living in San Jose and working for an oil refinery. People would call for room service at the hotel where he worked, just to have their photographs taken with him.

He grew to view talking about Kennedy’s assassination as a responsibility, a stand-in for the sadness of the entire country.

When Chris Fields is called for an interview, he doesn’t need to ask what it’s about. He already knows: It’s what people always call him for.

Fields was the Oklahoma City firefighter. The one who held Baylee Almon, who would have turned 19 last week if she had survived the bombing that killed 168 people. Still working for the fire department, Fields is a stoic-sounding man, not overly emotional or prone to sentimentality.

He didn’t used to understand why people made pilgrimages to the site of the bombing, or why they would feel such a connection to him based on a photograph. Then in the late 1990s, he went to Hawaii for an international firefighting conference and visited Pearl Harbor. He toured the site, he looked at the displayed images and he remembered the photograph of the men raising the flag at Iwo Jima.

He was moved to tears.

Moving forward

“That’s neat, isn’t it? What a picture.”

Bill Iffrig has been reached by telephone, shortly after his flight from Boston landed in Seattle on Friday. He hadn’t seen the famous photo until just a few hours ago. He’d seen on the news the widely circulated Vine video of him falling, and he knew that a lot of people wanted to interview him. But he hadn’t seen the iconic photograph until a gate agent at the airport pulled him aside and said, “I have something for you.”

He doesn’t quite know what to make of it, or of his new place in history. “I’ve recovered, a little bit,” he says. “I’ve lived with it for a week now.”

When the photograph was taken, what he was most preoccupied with — not understanding the breadth of the situation — was finishing the race. “After you finish the 26 miles, you really want to finish the last section,” he says, explaining why he dusted himself off and kept jogging, through the finish line and then the six blocks back to his hotel.

Which might end up being the iconic image after all. Not the idea of being felled to the pavement, but the idea of what came next. Finishing the race.

Ellen Rudolph is an international photojournalist in Tennessee. She is also a family therapist with a doctorate in counseling psychology. So when she thinks about images of tragedy, she’s thinking about both what makes a good photograph and about what makes a healing one.

What we want, she says, is something “that shows humanity and tragedy all at once.” Something that makes us remember the event, “but that also reminds us that there is good in life, and that evil is transient.”

The Fallen Runner, running on.