LONDON—A Banksy painting showing a boy playing with a toy nurse as a superhero sold for more than $20 million on Tuesday, setting an auction record for the elusive British street artist. “Game Changer”, unveiled last May at University Hospital Southampton, paid tribute to the frontline workers of Britain’s National Health Service (NHS) in their fight against the COVID-19 pandemic. The black-and-white hand-painted artwork shows a boy lifting a nurse, her arm outstretched and wearing a cape, while traditional superheroes Batman and Spider-Man lie in a bin. Through Southampton Hospitals Charity, proceeds from the sale will be used to “fund wellbeing projects for staff and patients, and distributed to a wider community of healthcare providers both within the NHS and charitable sectors,” Christie’s said. A reproduction of the painting will hang in the hospital, it said. In a Christie’s auction streamed live, the painting sold for a hammer price of …