Yours should look better than the one above, I was just in a rush.

First, open up your image and background you wish to put it on. (If you want the background I'm using, click here). Take your polygon tool and outline the person/sim you want mirrored. 

Right click on them > Copy > Paste on your background > Right click, validate paste. Right click again on the background, and paste your sim again. BEFORE VALIDATING PASTE right click and select "Flip horizontal" and move them as you wish.