Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a tear-jerker with people saying things like "the Lord moves in mysterious ways" and talk about seeing a "light at the end of the tunnel" when you're dying, and nobody gets too freaked out when a faith-healer fixes a broken back. The snake scenes are at the beginning and the end so I'm going to have to explain much of the plot, so you have been warned.
Ellen Burstyn was nominated for an Oscar for her perfomance here as Edna, a woman who lost the use of her legs when her car crashed over a cliff killing her husband. After a hospital stay she moves from Los Angeles back to Kansas with her father. Along the way they stop at a gas station in the desert somewhere east of Needles. The attendant asks her if she wants to see a two-headed snake for 10 cents. Of course she does, who wouldn't? So he brings the snake to Edna, telling her its name is Gemini. He tells her that it just crawled out from under his front porch one day. He also tells her that the Hopi Indians say that The Snake is a symbol of Mother Earth. She pets the snake's heads and drives off to Kansas.
After considerable effort she manages to heal her nerve-damaged legs and start to walk again. Then she starts healing others of their afflictions including nose-bleeds, stab wounds, fused vertebrae, and deafness. She is sent to a scientific institute in Calfornia where they wire her up and test her faith-healing abilities and confirm that they are real. She can even bend a laser light with her power. She doesn't know exactly where the power comes from - some say God, some say the Devil, some say it's from the psychology of those she is healing, but she thinks it is from the power of love.
Edna gets involved with Cal (Sam Shepard), the young son of a fanatically religious man who tells Edna that she is doing the Devil's work. Cal is no longer religious but after watching Edna heal people, he becomes religious again and tells her she's "the resurrection - the living Christ." She denies it so he tries to kill her. That convinces her to leave Kansas and we don't see her until many years later when her hair is completely gray and she's running the same gas station where she saw the two-headed snake. An RV drives up with a young boy who is dying of cancer, so she takes him inside to see the snake, which is now dead and preserved in a jar, then she apparently heals him.
The moral of the story, as I see it, is - be nice to snakes and you might get superhero powers, but I suspect others will see it a bit differently.
The live two-headed snake we see is a Pine Snake. The preserved two-headed snake in the giant jar is a Gophersnake, which is related to the Pine Snake, but looks nothing like it. But most people wouldn't remember what the first one looked like that much later.