Albert Barnes Commentary 2 Kings 11:18

"And all the people of the land went to the house of Baal, and brake it down; his altars and his images brake they in pieces thoroughly, and slew Mattan the priest of Baal before the altars. And the priest appointed officers over the house of Jehovah." — 2 Kings 11:18 (ASV)

A temple had been built to Baal at Jerusalem itself by Athaliah, Ahaziah, or Jehoram. According to Josephus, it was constructed in the reign of Jehoram. Its exact position is uncertain.

Images - The word used here is not the same as in 2 Kings 10:26, but a word which implies likeness. The Phoenicians had fashioned images, besides their unfashioned pillar-idols.

The priest appointed - Since temple worship had been discontinued during Athaliah’s rule, it now fell to Jehoiada to re-establish it. He had already summoned the Levites from all the cities of Judah (2 Chronicles 23:2) and had made use of them in the day’s events. Therefore, he immediately proceeded to assign the custody of the temple to a particular course before conducting the young king to the palace.