Bible story
Daniel in the Lions' Den Explained: Daniel 6 and Rubens
In Daniel 6, the Persian king Darius is talked into a decree that no one may pray to any god but the king for thirty days, and Daniel keeps praying three times a day, so he is thrown into a den of lions. Rubens' painting at the National Gallery of Art shows Daniel at dawn among the lions, looking up toward the light.
Updated
The painting

Sir Peter Paul Rubens
Daniel in the Lions' Den, c. 1614/1616
Oil on canvas
National Gallery of Art, Washington, DC
Public domain (NGA Open Access) · museum record · credits
Accession 1965.13.1
Daniel 6:22
The Gallery's audio guide notes that Rubens could study real lions in the royal menagerie in Brussels.
My God hath sent his angel, and hath shut the lions' mouths, that they have not hurt me: forasmuch as before him innocency was found in me; and also before thee, O king, have I done no hurt.
Daniel 6:22 (KJV)
The story
Daniel is one of the top officials under King Darius, and his rivals cannot find any fault in his work. So they persuade the king to sign a decree: for thirty days, anyone who prays to any god or man except the king goes to the lions (Daniel 6:7). Daniel goes home and prays three times a day with his windows open toward Jerusalem, as before (Daniel 6:10). The king is trapped by his own law and has Daniel thrown in, saying he hopes Daniel's God will deliver him. He spends the night fasting and hurries to the den at dawn. Daniel answers with the verse above.
The context
The Book of Daniel is set during the exile, when Jews lived under Babylonian and then Persian rule. Its stories ask how to stay faithful to God while serving a foreign king, and Daniel does both: he serves Darius loyally but will not stop praying. The famous image of the lions is really a story about a quiet daily habit that he refuses to give up.
Look at the painting
Rubens paints the den as a stone pit with an opening to the sky. The lions are life-size and lie and prowl all around Daniel, but none touches him. Daniel sits with his hands clasped and his eyes turned up toward the opening, where the morning light comes in.
Sources
Verse text from public-domain translations, quoted exactly with the reference. Paintings are named with their museum. Candlelit Bible is independent and not affiliated with any church or museum.