Smite evil still requires attack rolls. Mirror image provides a chance that an attack could miss. So the paladin's smite might not land on the first try. But smite evil no longer is wasted if the paladin misses; check the Core rulebook for a pleasant surprise :) Sooner or later, the wizard needs to stop the paladin before his images run out, or else he'll receive his righteous smiting!
It's still a valid question if the target is one of those things where the smite does double damage on the first successful attack. I'd say that hitting a mirror image is not successful, so doesn't waste the double damage.