Denis R. McNamara, an architectural historian who specializes in American church Architecture, holds a Ph.D. from the University of Virginia. He is the assistant director and faculty member at the Liturgical Institute of the University of Saint Mary of the Lake / Mundelein Seminary, and serves as a liturgical design consultant. His recent book, Heavenly City, was published by Liturgy Training Publications, Chicago. Duncan Stroik is director of the Institute for Sacred Architecture and a Professor at the University of Notre Dame School of Architecture.
While it is not forbidden to display an image of Christ, Mary or a saint behind the main altar, in modern churches this is usually reserved for the church’s patron. At the same time, the apse may be decorated with murals and mosaics figuring several personages. Therefore, I would say that the image of Divine Mercy would not normally be set up behind the main altar unless the church was dedicated to this devotion.