When you mean mirror image, are you meaning to flip it horizontally (i.e. an image of a face looking to the right would look to the left)? If so, and you're using Photoshop as MickS mentioned, you'll want to do a flip canvas.
Image > Rotate Canvas > Flip Canvas Horizontal
This will give you the same image, just flipped around as if viewing it through a mirror.
To my knowledge, the only "Inverse" function in Photoshop is a color invert, which gives you a photo negative version of the image you're wanting.