Mirrors are a nifty element to use on covers. You can create an easy ominous mood with just a blank mirror. There are some winner older covers where intrigue was made by having a different face in a mirror. Here are some of my favorites and what works with them.
The Megan Derr series makes beautiful covers with the same mirror backdrop
Mirrors wearing different faces of the protagonist
Mirrors with different people in the mirror
Mirrors with different effects
Mirror Effect covering entire cover
Rearview and Small Mirrors
Other Miscellaneous Mirrors