He forbids by Law the Placing his Likeness in Idol Temples
In this manner he represented himself, even through the medium of painting, as habitually engaged in prayer to God. At the same time he forbade, by an express enactment, the setting up of any resemblance of himself in any idol temple, that not even the mere lineaments of his person might receive contamination from the error of forbidden superstition.
Source: Life of Constantine (New Advent)