What does it mean when someone sends you a picture of a snake?
This question is unanswerable. A snake can mean a lot of things. For example, it is a symbol of healing. Think of the symbol of medicine; it is a snake wrapped around a cane. Also traditionally, in Greek and Roman mythology the snake was associated with the healing god, Aesculapius.
If we move to a different tradition, the snake represents deception or evil. Think of the story in Genesis 3, where the serpent in the Garden of Eden tricks Adam and Eve. They make them disobey God and this act plunges people in sin.
In light of these points, it is hard to tell what the person who sent you a picture of a snake means. It is best to ask that person directly.
This is an intriguing question. I have never before heard of a person sending another person a picture of a snake without some accompanying explanation. If I received a picture of a snake from someone I thought loved me, I would suspect that the intended message was that the sender no longer loved me. I might also suspect that I was being told symbolically that I myself was considered a snake for something I had done. I would feel a little apprehensive about anybody who used that form of communication. It almost seems like some kind of voodoo.