In some rare circumstances I think a mirror image might grant a flank bonus.
Reading the spell description, the spell doesn't spell out all the weird situations that can come up. A situation where an image 20 or 30 feet away from you is singed with a fireball, but you aren't, implies images can have a different appearance than you at any given time.
You can infer that images don't all face the same direction all the time, too. And as Jabba mentioned, being able to merge with an image implies at least a limited amount of conscious control over the images.
If the images drink when you drink and cast when you cast, you can infer they attack when you attack. It makes sense that an image 10 feet away might be in the position of a flanker to the opponent you're attacking. Whether illusions in general can effectively flank is another debate point I won't dive into here.
I personally would not make faux-flanking a feature of the spell, and would handle it on a case-by-case basis. I might require a Concentration, Bluff, or Spellcraft roll to give any bonuses. But, under the right conditions, I wouldn't turn a deaf ear to a well-roleplayed attempt.