speed

This is an interesting question. The problem is the definition of "image's speed". You could say that the image "exists" on the plane of the mirror. In this case the speed would be 0.75 meters per second since the image doesn't move.

If you mean the image's apparent speed, you would be walking in one toward the mirror in one direction, the image would "appear" to be walking toward you from the opposite direction. The speeds would add, and the answer would be 1.5 meters per second.