On a drizzly September day in 1997, Diana Vachier happened to be walking past an art gallery in New York when she spotted a painting that immediately struck her. It was entitled American Gothic and it had this great look about it: awesome colours, wonderful details and a style that seemed to just pop off the wall, in real pop art style.

The gallery owner explained that it was the work of an artist named Steve Kaufman, a former assistant to Andy Warhol. He showed her some other paintings of his, the subjects of which included Marilyn Monroe, the Mona Lisa, a Cohiba Cigar and a Beethoven. She thought they were fabulous. Just as she was leaving, the owner handed Diana about eight postcards featuring Steve's art.

A video editor by profession, and an avid lover of antique postcards, Diana put three of Steve Kaufman's postcards for sale on the internet, along with some of her old postcards. They were quickly sold.

A day or two later, she received an email saying: "Hi, I'm Steve Kaufman. Call me. I have a lot of postcards you can have." Rather nervously, since she felt out of her comfort zone, Diana still managed to dial Steve's number. A smooth, silky and very warm voice answered, and promised her that that he would send her a further 100 postcards that he would also sign. He then regaled her with some very funny stories about his life and girlfriends. At the end of a telephone conversation of some two hours or so, Diana was convinced that she had met a crazy, funny and brilliant artist.

Two weeks later a huge box arrived with more postcards than she could handle, all hand-signed. They were soon sold on the internet for $8 each and she kept a record of all her sales.

Diana and Steve were soon conversing regularly over the phone. He told her about his Give Kids a Break programme - a charity that helped young people that had just come out of prison. In order to raise money for his charity, he painted the paintbrushes he used to and sold them.

In the next few days, apart from the signed postcards, Diana was also receiving painted brushes and small mini paintings by the artist. The sales were great as her client base soared, and a lot of money poured into Steve Kaufman's charity programme.

One day, Steve asked Diana if she would quit her day job and work for him full-time. She agreed without much hesitation and pretty soon they were talking over the phone three to four times a day about marketing and promotions. She was on the east coast, while he was on the west coast.

Diana recalls how working with Steve was so much fun as they each bounced their ideas and goals off the other. She later wrote: "We were always laughing. In fact, he told me that the kids that worked with him always knew that he was talking to me because they would hear him laughing."

She recounted how, being on different coasts, it was easier to talk and share personal stories. "He would tell me about his family and girlfriends, his projects, and exhibitions and a lot about his dreams. He also told her how he had trouble sleeping at night because his brain just wouldn't stop working."

Diana warned him about his dangerous lifestyle and advised him to relax and take a vacation. He jokingly replied that the one time he had been on a holiday, he had suffered a mini stroke. Diana was also worried about his eating habits, because whenever he called, he would be eating some sort of junk food. And he couldn't be bothered to change his lifestyle because he was always glued to his paintbrushes.

In 2005, Diana and Steve they formed the American Pop Art Inc., built their licensing portfolio, placed Steve's art in several museums (with Coca Cola putting four paintings into their World of Coca Cola Museum) and started making inroads into the international markets.

In 2007, Steve had a second stroke but instead of slowing down and eating more healthily, he continued to work with even more energy. Two years later he had a serious stroke that did tremendous damage. He could not walk without a walker, lost the sight in his right eye and his hands shook uncontrollably. Even then, he would not stop. He would give talk shows in his wheelchair and still - miraculously - managed to paint and produce beautiful art.

On 11 February 2010, Steve Kaufman flew to Denver to give a speech at the opening of one of his exhibitions, although almost everyone he knew, including Diana Vachier, warned him not to go. The next morning Diana received a call to tell her that he had passed away during the night. He was 50 years old.

Among Kaufman's more recognisable images are those of Marilyn Monroe, Frank Sinatra, the Beatles, Elvis Presley, B.B. King, Sammy Davis Jr, Al Pacino, Muhammad Ali (Cassius Clay), President Barack Obama, Jay Leno, John Travolta, mobster John Gotti, Beethoven, Napoleon and such icons of Americana as the $100 bill, Coca-Cola images and Campbell's Soups art - some of which have been wrongly attributed to Andy Warhol.

Following his death, Diana redoubled her efforts to introduce Kaufman's art on the international stage. Amongst the collection of art she inherited from Steve was an anthology of Andy Warhol's most representative works. Diana's inspiration to hold exhibitions of these two great artists together paid rich dividends because it meant that, for the very first time, the world could now experience the birth and development - indeed the whole story and history - behind the great Pop Art Movement.