The left hand and the right hand are 'mirror images' (of each other). = The left hand is the/a 'mirror image' of the right hand.
He's the 'mirror image' [=(more commonly) 'spitting image'] of his father. [=he looks just like his father]
She saw her reflection in the 'mirror'. = She looked at herself in the 'mirror'.
(..) And she was devoted to him. He gave her everything, ... but time. Thank you, Mr. Dumbledore. He did save our lives twice! Kept an eye on us on that mirror. That doesn't seem like someone who's given up! She's coming back. What's that with her? Neville! (..)2011 Harry Potter and the Deathly Hallows: Part 2
(..) - Why've you left him? - He's dead. Sorry to hear it. I liked that elf. Who gave that to you? The mirror? Mundungus Fletcher, 'bout a year ago. Dung had no right selling that to you. It belongs to... Sirius... Albus told me. (..)2011 Harry Potter and the Deathly Hallows: Part 2