This fresco, one of several representations I have seen depicting the famous
Battle of the Milvian Bridge, between Constantine the Great and Maxentius in
312, is by Raphael Sanzio. (Another one that I like is the fresco I saw on the
ceiling of the church at Andechs, above the Ammersee, not too far from Munich).
It is difficult to ascertain just what really did happen. In our day and age it is not so easy to accept the idea of a real, genuine visitation of this sort; it is far easier for us to think in terms of a psychological or spiritual event - or perhaps even a dream stimulated and provoked by the situation, which is certainly more credible to our modern concepts; (Lactantius, an early Christian scholar and tutor to Constantine's son who was therefore close to Constantine and certainly must have had several opportunities to question him directly had he wished, suggests, in fact, that it was a dream: "Constantine was directed in a dream to cause the heavenly sign....."). As John Julius Norwich* - and other historians point out, there were nearly one-hundred thousand soldiers present as "witnesses" to this remarkable occurrence and yet it was never mentioned - not by Constantine himself, nor by anyone else who was present; no contemporary historian or writer comments on it - other than the report by Lactantius mentioned above, and it was only when Eusebius wrote his biography "De Vita Constantini", much later, that the phenomenon has its first mention in a supernatural context.
Very few of the representations of the Battle of the Milvian Bridge show the "cross" as it was used in the east, and as we know it to have been in those years. It certainly was not the familiar Latin one that we know; rather it was composed of the first two letters of the word "Christ", written in the Greek alphabet: Chi and Rho, (Chr[istos]) - X with the Rho (P) superimposed across the centre. Some depictions of this symbol show the "X" in the vertical position as a "+" with a loop added at the end of the upright to turn into the "Rho". In any event, Constantine adopted the symbol (known as the Labarum), and had it placed on his troops' shields and used it thereafter as a standard.
Interestingly enough, Raphael's fresco shows the inscription of the usual, well-known Latin phrase in hoc signo vinces in Greek. The complete phrase is: en toutoi nika - in this, win! - the word "NIKA" for "victory" or "win" being clearly seen. It is a little strange that he should have decided on this anachronism, because Latin did not give way to Greek in the east until late in the fourth and on into the fifth century, nearly a century after the event.