On 11 September 2001 we witnessed one of the most horrific news stories unfolding before our eyes – the attack on the World Trade Center in New York. More than 2,000 pictures flooded in to the Guardian picture desk, scenes so extraordinary that they were like stills from a disaster movie. They came from photographers, journalists and ordinary people caught up in the chaos, taken on cameras, as stills from TV broadcasts and on mobile phones.

More than 16 years later, around 30,000 images arrive at the picture desk in any 24-hour period. Every one of them is viewed to see if they should make the cut for our daily edit. It can seem a daunting task, but we tackle it forensically because I never want to see a picture we’ve overlooked appear on the front page of another paper.

Pictures are pinged in from all over the world, some just seconds after being taken. They are provided by the global news agencies we subscribe to, local agencies, individual freelances and our own commissioned photographers. The speed between the shutter being pressed on the camera and the image being published online can be startling: I used to take a deep breath when an editor asked for a picture they had just seen on a TV news bulletin, expecting to get it immediately, but in this digital world that’s almost a reality.

This has transformed the way we make picture choices for online stories. There can be a tendency towards hasty publication, to be first with the news, but it pays to pause for a moment to verify sources and make certain that what’s being portrayed is real and accurate.

While pictures are dropping in, stories are being written that require original photography, so there’s a constant stream of images being commissioned. This isn’t just a UK operation; we commission photographers from Lima to Jaipur, and they will be expected to understand our editorial philosophy, shoot to the standards required and file to the given deadline.

Online, we have the opportunity to create handsome visual displays, so a commissioned photographer is encouraged to offer a selection of images for any given story. When pictures speak louder than words we have also developed an elegant photo-essay format to display longer-form pieces and documentary series.

At midday, the section editors for the print newspaper meet to review the major stories and begin working out the flow of pages. It’s here that the picture desk presents a slideshow of the best and most useful images of the day so far, including suggestions for one of the most challenging slots from a picture editor’s point of view – the double-page spread Eyewitness.

When Eyewitness was first introduced in our previous redesign (some 10 years ago) it seemed like a daunting space to fill with a single image: not every photograph submitted is of sufficient resolution to be reproduced at such a size. Any image selected must also be topical and deserving of this huge amplification over all the other pictures in the newspaper.

Eyewitness has captured moments of quirky daily life around the globe and epic scenes of natural wonder, from the splendour of cultural festivals and celebrations to the heartache of human crises and conflict. On a good day, the successful contender is in place by early afternoon, but if nothing’s quite working then deadlines dictate that we can hold our nerve until 5pm(ish). However, I find not securing the image early in the day can be preoccupying, so I often have a couple of useful standbys squirrelled away.

One thing an upbeat Eyewitness image can deliver is a moment of relief, if the news run that day is particularly grim. Last year we found ourselves caught in the most tragic news cycle, with a succession of terror attacks and the awful fire at Grenfell Tower, and only last week there were shocking scenes of violence in Gaza – graphic images of the dead and wounded accompanied by distressed friends and families. I’ve been editing news pictures for more than 20 years and I’m still affected by upsetting images, sometimes to the point of tears.

Developing ways of coping with this is important, and as a team we encourage an open dialogue about it. I believe acknowledging our human response is vital in helping to inform a decision over what to publish or, just as importantly, what not to publish.

The decision to print a picture that shows scenes of graphic violence, dead bodies or distraught relatives is never taken lightly. Each image is considered within its own news context. We question whether dignity has been compromised, the level of intrusion, and whether publishing such an image would be gratuitous or sensational.

One of the most affecting and debated images published on our front page and online was the photograph of the child refugee Alan Kurdi, lying dead on a beach in Turkey. The story of refugees making perilous journeys to reach Europe was already a long-running one by then. Social attitudes towards the rising humanitarian crisis were negative, and political action was inert. It seemed clear that such a shocking image might be a tipping point, with the potential to effect change.

Interestingly, other editors reached similar conclusions and, while it’s very hard to measure this sort of outcome, what followed appeared to be a transformation of spirit: the language surrounding the story changed, donations flowed in to aid agencies, and politicians realised they had to act.

While the front page picture is the most important one, it’s usually the last piece of the daily puzzle to fall into place. It doesn’t have to be a literal illustration of the main story, but it should be newsy and dynamic enough to stand out from the crowd of competitors on the newsstand. I still marvel that, with thousands of images to choose from every day, newspapers frequently carry identical front page pictures or images featured in daily online best-pics galleries. Yet the front pages I’ve been most proud of have been made on days when we’ve made a different choice and taken a risk to create an original, eyecatching cover.