Shout after him.

There's no response from the man, but your outburst seems to have attracted the attention of his pursuers. Two swarthy thugs appear, armed with very big knives.

One of them gestures towards the painting. "Hand it over", he orders.

You can:

Run like hell in the opposite direction. They're out of breath, they'll never catch you.

Try to fight your way past the leading hood.

Meekly give him the painting and hope he won't hit you.