This image is from a Wikimedia contributor named AnonMoos (See other images from his gallery here.)
According to Wikipedia it comes from a 13th century Biblical text called the Douce Apocalypse, citing The Heraldic Imagination by Rodney Dennys as its source. I can not independently confirm this, and I love Wikipedia like you love the drunken uncle who has all those stories, but you don't necessarily believe all of them. Supposedly the images come from the Book of Revelation:
And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared.
And I saw three unclean spirits like frogs [come] out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet.
For they are the spirits of devils, working miracles, [which] go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
The website of the Library of the Medieval Institute at Notre Dame has some wonderful, creepy images of the Douce Apocalypse. Learn more about the End Times here, kids.