Abandon it.

You lean the painting against a convenient wall, and walk causually away. Moments later, a tall, heavily-built stranger jumps out in front of you.

He waves a large, bloodied knife at parts of you where you don't want it to go. "Hand it over," he says.

You can:

Look behind you in case his accomplice isn't there holding an equally big knife.

Try to fight your way past him.

Smile, shrug, shake your head, and repeat a few foreign phrase you remember from the opera. Hey, it's worked once.

Say, "I left it propped up against that wall back there".