Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Clark Gable is a cynical tough guy convict and Joan Crawford is a bitter saloon girl in this strange movie about spiritual redemption and good and evil, but they are not involved in the snakebite part of the plot. Several prisoners escape Devil's Island prison and slog through the jungle towards a getaway boat. One of them named Telez wades through some water and rests on the shore with his legs in the water. He was whacked on the head and left for dead, so he has good reason to be exhausted. But as he lies down we see a snake swim up to him and he winces in pain. (The snake, mostly just a moving presence in the film, is hard to see in still photos where it is only a bump or a ripple in the water.) Later we learn that the snake bit Telez on the leg. This is a good illustration of the popular misconception that a snake will bite anybody anytime for no reason since Telez presented no threat and nothing to eat for the snake. To make the situation even less believable, Telez somehow survives through the night then crawls with his hands through the jungle to meet up with the other escapees on the beach. The fugitive Dufond, who says he was a medical student long ago, takes a knife and cuts the wound to lance the swelling and the mysterious stranger Cambreau, the all-knowing self-sacrificing Christ-like figure whose words and actions embody the movie's message, sucks the poison. Of course, even if the cut and suck method did work for snakebite, after so much time, the poison would be long gone from the site of the bite where it first entered Telez's body. The convict named Hister knows this when he tells Cambreau that the treatment will not work and Telez will be dead in the morning, and sure enough, he is. This is the only realistic aspect of the snakebite part of the plot.
I don't know how they achieved the snake swimming in the water effect, but I don't think it was a real snake, though I can't say that for sure. I'd guess that they manipulated a fake snake somehow, maybe by pulling it along with wires to make it look like it was swimming. Just before it swims to Telez's leg, it changes course slightly towards him, making it look even more like the bite was not accidental, although I suspect this course adjustment was simply caused by the method with which they moved the snake.
An IMDB trivia comment states that during filming Joan Crawford was so startled by an eight-foot python coiled on a branch above her that she refused to repeat the scene. This might be why we did not see a python in the movie. It could also be a made-up story used to promote the idea that "mommy dearest" Crawford was a difficult prima donna on the set.