Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Louis Bunuel was a brilliant Spanish filmmaker. This was his first English language film, but it was made when he was making films in Mexico. It's based on the classic novel by Daniel Defoe, published in 1719. The snake scene is very brief.
On a journey to buy slaves in Africa, Robinson Crusoe is shipwrecked on an unknown island. Good riddance slave trader, the end. Not exactly - nobody cared about that in 1719. Anyway, we see Crusoe learn how to survive on the island for most of the movie. In the end, he has lived on the island for 28 years when he and his companion Friday, who Crusoe rescued from cannibals, discover that a band of mutineers have rowed to the island with their ship's officers in bondage. As the mutineers fight off a bunch of cannibals, Crusoe and Friday free the officers. The mutineers follow them all to Crusoe's fort to search for gold. While ransacking the fort, one of the mutineers discovers a snake in a box full of grain. He pulls out his pistol and shoots it (because the only good snake is a dead snake.) Then he continues with his pillaging.
The snake we see is a live snake. I think it is some kind of Mexican rattlesnake since the film was shot in Mexico. As far as I can tell, they actually killed the snake for the scene, because we see it without a head.