When celebrated artist Jonathan Yeo met Malala Yousafzai, just six months after she had been shot by the Taliban, he thought the teenager would be frail. But he hadn’t expected the education campaigner to be as quite fragile - or as fun - as she was.

“She had a big metal plate in her head,” he said. “Walking was a lot of effort. But she’s a very positive person. It doesn’t take very much for her to see the funny side, to be cheerful.”

Mr Yeo, a renowned portrait painter, had two sittings with Malala in 2013, which involved not only sketching the teenager but also having lunch with her and her family.

The resulting picture is being auctioned by Christie’s in New York City on 14 May, and is expected to fetch up to $80,000 (pound;47,000) for the Malala Fund, which was set up to further her work fighting for girls’ education.

“I was very struck by how frail she seemed,” Mr Yeo said, “especially for a 15-year-old - they are usually pretty sprightly. She had only been out of hospital for a few weeks and had started school and was in an extraordinary situation.

“Not only did she have these dramatic physical changes but she also faced cultural ones, too. And there was a lot of pressure on her from people treating her very differently. But she was still cheerful and acted as if these things were all vaguely normal.”

The portrait was previously exhibited at the National Portrait Gallery in London as part of a retrospective of the artist’s work. It shows Malala sitting sideways at a table, holding a book and looking over her shoulder at the viewer.

“The National Portrait Gallery contacted me about this retrospective and wanted some new works for it,” Mr Yeo said. “I had painted a few politicians and I was interested in the idea of mixing it up, of painting people who represented a new kind of politics, who did not necessarily go into politics by choice but who had it thrust upon them.

“Malala and Doreen Lawrence [now Baroness Lawrence of Clarendon, the mother of Stephen Lawrence, a teenager murdered in a racist attack in 1993] seemed the obvious ones and I was lucky I had a positive response from both of them.”

Malala, now 16, was shot as she travelled home from school in Pakistan on 9 October 2012. She was attacked for speaking out against the Taliban, which had banned girls’ education as well as closing and destroying schools.

“I think it’s important what she’s doing,” Mr Yeo said. “The very act of painting, of creating an icon of someone who deserves it, is an important part of my job. Having made that decision, to then decide to support her organisation was logical.

“There is a quiet power to the portrait, which I think is what she has. You can’t get every facet of someone’s very complex character, but you hope to allude to different aspects of them. Someone of that age changes very quickly. It looks like her now but maybe not as she’ll look in two or three years’ time. It is really of that moment, which is a nice thing because it was a pivotal time in her life.”

A review of the portrait on website Theartsdesk.com called it “striking.simple and powerful”, but it was described by The Telegraph as having “an almost Victorian sentimentality”.

Malala merchandise - not always sold for charity - now includes mugs, posters, fridge magnets and T-shirts. And Fife Council in Scotland even has plans to distribute desks bearing her name to every school in the authority.