Although you might need to consult with a lawyer to determine issues of employee rights, it would seem to me that issues of mere consideration would entail whether or not the image is displayed in such a way that colleagues and clients cannot help but see it. If your employee works in a personal office or cubicle and does not interact with the public, then there seems to be no reason to require her to relocate her picture. But if she works in an open office space (where her desk is visible to colleagues or their clients all over the room) and others cannot reasonably be expected to avoid the image if it disturbs them, you might suggest to your employee that she either keep the large image in a desk drawer (from which it can be brought out for meditation on personal time) or substitute a small holy card of the Crucifixion that will be more discreet.
