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 surrealist movie made by cult filmmaker Alejandro Jodorowsky, whose movies Santa Sangre and The Holy Mountain are also on my snakes in movies list. I won't claim to understand why the snakes are used in this movie. Jodorowsky seems to usesnakes as symbols the same way he uses corpses coming out of graves, dolls, people with severed arms, and other strange symbols which may not mean anything at all and that each viewer will probably interpret differently.
Among the many very strange things we see in this movie is a man cutting a hole in a doll, taking a handful of juvenile garter snakes out of a jar, and trying to stuff the snakes inside the hole in the doll. Then Fando grabs a handfull of them and puts them on the doll before he runs away. If that blatant sexual symbolism is too easy for you, the next appearance of the garter snakes is less obvious, to me at least. We see a bunch of juvenile gartersnakes on the white grave marker of Lis, the woman of the duo Fando and Lis. When Fando arrives most of them are gone and he clears the others away and puts a flower on the marker.
The movie was made in Mexico, so I would assume the snakes used here are one of the species of gartersnakes found in Mexico.