For what it's worth, the 3.5 Main FAQ has this to say:
The mirror image spell description says the images have
an Armor Class of 10 + size modifier + Dexterity modifier.
Can you improve this with spells the spellcaster casts on
herself, such as shield or mage armor? If so, why doesn’t the
spell description say the images have the caster’s Armor
Class? What happens if the caster has cover from her
surroundings? Will cover improve the images’ ACs? What
about concealment? Will fog or foliage produce a miss
chance for a foe that aims an attack at an image? What
about magical concealment, such as a blur or displacement
spell?
The images from a mirror image spell don’t use the caster’s
Armor Class. Use the formula in the spell description to
calculate each image’s Armor Class (10 + caster’s size modifier
+ caster’s Dexterity modifier). Use the caster’s current
Dexterity modifier for each image’s Armor Class, no matter
how the caster happened to get that modifier. Any Armor Class
improvements the caster might have from equipment she
carries or wears, or from magic operating on her person, don’t
apply to the images. For example, a Medium user with a
Dexterity score of 16, a shield spell, and a suit of +2 leather
armor has an Armor Class of 21 (10 +4 shield, +4 armor, and
+3 Dexterity), but her images have an Armor Class of 13 (10
+3 Dexterity).
It’s easiest to assume the images share the user’s location
on the battlefield, and gain any cover bonuses that might apply
to the spell user in that location. If the character in the previous
example were behind cover, she would have an Armor Class of
25 and her images would have an Armor Class of 17.
If the user has concealment from her surroundings, the
images have the same concealment. The images also look just
like the caster, and they share purely visual effects such as the
blur or displacement spell. If the mirror image user is also
using either of these effects, an attack aimed at an image has
the same miss chance an attack aimed at the caster has.
What happens if a mirror image user is incorporeal? Are
the user’s images also incorporeal? Do attacks aimed at the
images have the incorporeal miss chance? If the incorporeal
user moves through a wall, can the images move through
the wall, too? What happens if the user goes to another
plane? Do the images go along? What if the mirror image
user employs a blink spell?
Incorporeal spell casters create corporeal effects. So the
figments from an incorporeal user’s mirror image spell are
themselves corporeal. Attacks aimed at the images have no
incorporeal miss chance.
The images, however, appear like the caster and move as
the caster moves. If an incorporeal user moves through a wall,
its mirror images also appear to move through the wall.
If a mirror image user moves to another plane, the images
go along. If the user also employs a blink spell, the images
blink right along with the user, and any attack aimed at an
image has the same miss chance (50%) it has if aimed at the
caster.
-Hyp.
