The image is congruent to the original figure. The translation, rotation and reflection operations do not change the size or shape of the figure. It just moves it around in some fashion. Think of it like this: imagine you had a camera and you were aiming it at a triangle. If the triangle holds still, but you move the camera, then the viewer (watching it later) will see the triangle move. In reality, it's the camera moving of course. The triangle does not change its size or shape as the triangle isn't moving at all. This idea can be applied to reflections and rotations as well.
