Aaron Intercedes for Miriam
Vatopedi Monastery, Mount Athos, Greece
This is an illustration of Numbers 12:1-15. Miriam and Aaron have confronted Moses, asserting their equality with him as prophets. So God calls them to the meeting tent, confirms Moses' primacy, and in his anger leaves Miriam "all covered with leprosy." Aaron then asks Moses to intervene on her behalf, and when he does God relents and says Miriam can simply stay outside the camp for seven days.
The meeting tent is on the left. Moses is in the middle and Miriam on the right, her face and hands covered with the marks of leprosy. Unlike her brothers, she has no halo. Aaron is in the center, wearing the trousers and buckled cape often seen in his portraits. The hand reaching down from the cloud above Aaron is the traditional way of representing God without actually picturing him.
It is almost certain that the image is not of "Moses confronted by Miriam and Aaron," as the Wikimedia source would have it. That confrontation occurs before God calls the three to the meeting tent, and Miriam does not contract leprosy until after God has withdrawn from the tent.
Read more about images of Moses.
Read more about images of Aaron.
Read more about images of Miriam.
Source: this page at Wikimedia Commons.