Correct me if I'm wrong, but :
The spell Mirror Image makes it impossible to target the MI's caster dirrectly, which means you cannot, say, target the caster with a Dispel Magic, meaning that if you want use a Dispel Magic on the spell's user for a Targetted Dispel, you must get rid of the Figments first ( or try your luck and target one of the images - same would apply to any other non-area spells ).
The spell Mirror Image makes it impossible to target the MI's caster dirrectly, which means you cannot, say, target the caster with a Dispel Magic, meaning that if you want use a Dispel Magic on the spell's user for a Targetted Dispel, you must get rid of the Figments first ( or try your luck and target one of the images - same would apply to any other non-area spells ).