Twice in Mark (with parallels in other gospels) Yeshua confronts hypocrisy about idolatrous coins. The issue of coins containing symbols of foreign worship (avodah zara) came up in an early rebellion against Rome in 6-7 CE (Horsley and Hanson, Bandits, Prophets, and Messiah, pgs. 196-7).
The two conflicts of Yeshua involving idolatrous coins concern the Temple tax coinage and Yeshua’s protest action (Mark 11:15-19) and the entrapment question about paying the poll tax to Caesar (Mark 12:13-17).
The Temple Tax in Mark 11:15-19
The Temple state (chief priests mainly) required the Tyrian shekel to be used in paying the Temple tax of one half shekel per household. The Tyrian shekel was more pure in its metal content than other forms of coinage, but had an image of Baal Melkart on it (the Syrian Hercules) and was therefore idolatrous. The Temple state’s priority was not holiness, but commerce, power, and wealth (see Maurice Casey, Jesus of Nazareth, pgs. 413-15). The money-changers were actually bringing the image of Baal or Hercules onto God’s Temple mount.
The Poll Tax in Mark 12:13-17
The poll tax was one of various forms of taxation imposed by Rome and involved each man, free or slave, paying a Roman denarius (had to be that specific coin) as a combination of a census and a tax (the number of coins indicating the population). The Roman denarius had an image of Caesar, already thought by many to be an idolatrous image due to the Roman imperial cult, and said on it pontifex maximus (highest priest) and DIVI AUG[ustus] F[ili] AUGUSTUS (son of the deified Augustus, see Casey pg. 423). The Pharisees actually ganged up with Herodians to entrap Yeshua and their hypocrisy was exposed when Yeshua had a Herodian in the crowd produce a coin with the Caesar as god image on it.