As far as I can tell, most targetted spells will say Target: One or more creatures, but as far as I know the intent is, if you try casting a targetting spell (like Charm Person) on someone with Mirror Image then you pick an image and cast. If it's not the real creature, then the spell is lost, but the image is also destroyed.
Hypersmurf said:
Ah. Message, the ultimate anti-Mirror Image spell.
Level 1 spell. Targets: 1 creature/level.
-Hyp.