Firmly inform the driver that the painting is
nothing to do with you
and that if he doesn't clear off you you'll call the police.
Take the painting with you
and accept your fate.
Just as you think you've got away with it, the taxi driver comes running after you holding the painting.
"The things people leave behind in cabs", he smiles, his eagle eyes approving the further tip you feel obliged to place into his expectant hand. "I 'ad one bloke left a wooden leg behind once. Wasn't his, neither."
You can:
Firmly inform the driver that the painting is
nothing to do with you
and that if he doesn't clear off you you'll call the police.
Take the painting with you
and accept your fate.