Why an icon not a painting?

“The wider significance of the events portrayed [in the icon] has to be worked out in the soul of those who behold the icon; what the icon represents may have been manifested at a precise point in time and space, but its fuller significance is found in the inner world where the true work of purification, illumination and union have to be accomplished” (Fr. John Baggley, 1987, Doors of perception: Icons and their spiritual significance, p. 82).

More about why it was an icon, not a painting, that was requested by God the Father for consecration and veneration can be found here.