This is probably obvious ot a lot of people, but might be useful information to some.
As far as I know, Mirror images snapshots the stats of the casting mage when they are summoned. This means that if you maximize stats that are useful to the mirror images when you summon them, they will do more damage. They benefit from haste, crit and spell power.
This means I don't cast Mirror Images pre-pull anymore. Generally I wait until Alter Time expires (to get the current 2-set bonus active on top of everything I had stacked up for AT). Later on, Tempus Repit (meta gem proc) is definitely one of the more powerful buffs to watch for using MI. As frost, Mirror Images lines up nicely with Icy Veins and Alter Time.
On Twins last night, my first set of mirror images did 631k damage (hero later in the fight). On Iron Quon, the first cast did 774k (early heroism). Not bad for a GCD, I guess.