The moment when the "Lamb" and the "Lion" lay down together is the time of His great return. He is both. A Lamb to those who are prepared at His coming. A Lion to those who are not prepared, for whom judgment will be poured out.
When you see that painting of the Lamb and Lion lying down together (we have one in our Stake Center), you are seeing the two great symbols of the Lord's Millennial reign.