I'd rule that the magic missiles fly unerringly where aimed (i.e., the "target"). That means the missile(s) would automatically hit whichever image aimed at (real or mirrored). Since the spell description allows multiple missiles to be split up against multiple targets I'd allow a caster to do so, meaning I'd roll for each missile separately, with the chance of hitting real vs. mirror image increasing with each dissipated mirror.

Since the missiles can be aimed at multiple targets, I would probably allow a clever player to make the following argument: "I see four versions of the enemy sorcerer in front of me. I can unerringly fire four magic missiles when I cast the spell. Therefore, I'm going to aim one missile at each apparent sorcerer that I see." I would then allow a missile to unerringly hit each of the three duplicates (dispelling it) and the final missile to unerringly hit the actual sorcerer (inflicting damage). The player would have to think of this argument himself though. If he didn't, I would randomly determine the target of each missile as usual.

Mirror image is a first level illusionist spell, so I think there are limits to how much it should be allowed to foil other spells.

I agree with the replies above; Magic Missiles strike the targets the caster points at, without any ambiguity from phrases like 'intended target' or anything.

But I read into it from first principles for myself as well – in the 3E rules; not going back to 2007!
   I think it might be worth pointing out Magic Missile's "instantaneous" Duration, give or take "instantaneously" pointing at multiple targets(!).  The caster doesn't have the time to see the effect of one missile before launching the next; and the missiles aren't in flight long enough for the interweaving Mirror Images to foil them.
   I slightly wonder at the multiple images using their DX bonuses to dodge missiles, and whether missing one ought to have a chance of hitting another!  But that's not the spirit of the spell, so let's not go full 'simulationist'

Cheers,
–Os..

Yes, @Rhialto, I shouldn't have said "to dodge missiles" when we were talking about Magic Missile, which absolutely does always hit.  I was getting carried away and thinking about ranged attacks in general.