It was one of the defining photos of this summer’s civil rights protests, and was all the more remarkable for being taken by a man who taught himself on YouTube after receiving a camera from his wife on his 39th birthday. This month Misan Harriman, a former City of London headhunter, became the first black male photographer to shoot the cover of British Vogue. On Thursday, Sotheby’s announced it would be sell his first print at an auction to raise money for the Anthony Nolan blood cancer charity. Harriman’s life has been a whirlwind since he travelled to a Black Lives Matter protest in central London in June. Near the US embassy he spotted 18-year-old Darcy Bourne by the side of the road with her placard which simply said: “Why Is Ending Racism a Debate?” He asked if she minded having her picture taken on the road. At the same time a protestor behind her raised a fist in solidarity. The resulting image went viral after being shared by figures including the Vogue editor, Edward Enninful, the athlete Dina Asher-Smith, the racing car driver Lewis Hamilton, and Martin Luther King III. Harriman, 42, told the Guardian he was drawn to the placard. “It’s a question that makes all of us reflect on why we haven’t asked ourselves this question sooner?” The day itself was an emotional one. “I remember thinking … I never thought in my lifetime I would see this many people from all walks of life fighting for my rights. “It’s a very strange thing, as a black man, to be at a protest where people that don’t know who you are, are truly and sincerely fighting so that you have a chance in life.” He added: “There was a sense of solidarity that I didn’t see reported widely. Because of that, there was a sense of hope that my children and their children’s children will never have to ask the question in that picture.” Bourne, an England under-21s hockey player, said she had only just written the sign. “The paint was still wet when the photo was taken,” she said. “I wrote that question because I’m in disbelief every day that we still have to fight for equality, and I wanted to remind people of the purpose of the movement.” Enninful, describing Harriman as “the voice that was missing”, commissioned him to take portrait photographs of activists for Vogue’s September issue, including Manchester United footballer Marcus Rashford and the model and mental health campaigner Adwoa Aboah. The respect is mutual. Harriman said of Enninful: “It is a great example of what happens when you have diversity at the very top of the food chain – it trickles down. It’s a lesson to many organisations that if you have someone who has that lived experience, of being othered, he will have the empathy and presence of mind to do the right thing in terms of empowering others from a diverse background.” Harriman approached Sotheby’s about selling a print to raise money for Anthony Nolan after a friend who had cancer told him about the charity’s work. The print will be on view at Sotheby’s London headquarters from 11-14 October while at the same time being offered at an online auction with an estimate of £3,000–£5,000. Harriman is a clear example of it never being too late to find your true vocation, although he believes he has been preparing for it his whole life. At school “when most boys were talking about football I was talking about the lighting in [the Stanley Kubrick film] Barry Lyndon.” • To buy the Guardian’s Black history wallcharts, visit the Guardian bookshop and use code 15CHART for a 50% discount.