God's commandment "thou shall not make a graven image" is entirely connected to the worship of false gods. (Deut. 5:8) God does not prohibit images to be used in worship, but He prohibits the images themselves to be worshiped. God commands the making of the image of a golden cherubim. This heavenly image, of course, is not worshiped by the Israelites. Instead, the image disposes their minds to the supernatural and draws them to God. (Exodus 25:18-22)
Then God also commands the making of the bronze serpent. The image of the bronze serpent is not an idol to be worshiped, but an article that lifts the mind to the supernatural. (Num. 21:8-9) If anyone who has been bitten looks at it, he will recover: "and as Moses lifted up the serpent in the desert, even so must the Son of Man be lifted up, that those who believe in him may not perish, but may have life everlasting" (John 3:14-15).
In Numbers 21:4–9. Attacked by a plague of snakes in the wilderness, Moses holds up a serpent coiled around a staff, both made from bronze, so that the Israelites might recover from the bites. That is where the serpent/snake on a staff comes from in medicine.