[Micah]

I'm pretty sure that's not how it works in practice because of defense penalties, actually.

From what I've seen mirror images and (I think) other defensive buffs cancel out the attack entirely, negating the multiple attack penalty and thus making images + high defense much more effective than a single negation since the mirror image gets to negate not just hits, but misses as well. This is based on anecdotal evidence of watching units with mirror images lose the images and die quite quickly, whereas they had been able to go for multiple rounds previously without a single hit, but I haven't done extensive testing.