6. And the ravens brought him bread and flesh in the morning, and bread and flesh in the evening; and he drank of the brook.
God tells the prophet Elijah to hide himself at the brook of Cherith. The drought that Elijah had predicted came true, and the brook was the only place in the land with water. Ravens supply Elijah with bread and meat.
The inscription on the stone, "Reg. III, Cap. XVII", refers to the third Book of Kings. Guercino probably had a Bible where 1 and 2 Samuel were still counted as 1 and 2 Kings.
Guercino painted the picture in Ferrara for Cardinal Jacopo Serra.