Let's consider a situation.
I am sitting in a room in front of the mirror.i see my image in front of me .I am in motion with respect to someone in space as earth is rotating. So is my Image also in motion?
if it is, then in whose respect? As no one apart from one sitting in front of the mirror can see my image.
Since the mirror is placed in earth , the image formed in the mirror is not in motion with respect to space because both image and men are in earth and there is no relative motion between men and mirror ,so image is at rest with respect to any medium , but men and mirror is in motion with respect to space . If there is no relative motion between object and mirror , there will be no motion in image with respect to any medium since .