Per Gorchestopher's comment, the above image is a deviantart (e.g. fanmade) picture by Stephen Howard showing a "Celtic Supergirl", part of a series of 73 "out of time" images suggested by fans.
On the subject of Superheroes in Ancient Rome, there have been some time-travel stories you may be interested in including "Lois Lane, Slave Girl!" (from Superman's Girlfriend Vol 1, #33). Lois is transported in time and sold as a slave.
"Challenge of the Superfriends - The Time Trap" in which Superman, Batman and Robin are transported to Ancient Rome to face fearsome lions (well, one arthritic-looking lion at any rate) in Caesar's gladiatorial arena.
And "World's Finest #198" in which a pyjama-clad Jimmy Olsen is sent back in time by Superman and the Flash's speed challenge. He faces a Centurion who (unsurprisingly) decides to kill him.
