In 3.5 for Major Image, a monster was summoned and someone charged it. If somebody charges for an illusion, what is the "to hit" on it? The description of major image is: "The image disappears when struck by an opponent unless you cause the illusion to react appropriately." What is the check for it to react appropriately?
No check need be made, and, really, that whole paragraph should be ignored, because...
That's Legacy Text
In previous editions of Dungeons and Dragons illusion spells had different names and different effects. In Advanced Dungeons and Dragons, 2nd Edition the basic illusion spell was the 1st-level wizard spell phantasmal force, which reads, in part,
This spell creates the illusion of any object, creature, or force, as long as it is within the boundaries of the spell's area of effect. The illusion is visual and affects all believing creatures (undead are immune) that view it. It does not create sound, smell, or temperature. Effects that depend on these senses usually fail. The illusion lasts until struck by an opponent--unless the spellcaster causes the illusion to react appropriately--or until the wizard ceases concentration upon the spell (due to desire, moving, or a successful attack that causes damage).
--From page 177 of the 1996 second printing of the Player's Handbook (1989) for Advanced Dungeons and Dragons, 2nd Edition
Emphasis mine. Note how much the italicized text resembles the closing text of the Dungeons and Dragons, 3rd Edition 3rd-level Sor/Wiz spell major image [illus] (PH 252), which reads, "The image disappears when struck by an opponent unless you cause the illusion to react appropriately."
No rules are given for how to make the illusion react appropriately in Dungeons and Dragons, 3rd Edition because while in Advanced Dungeons and Dragons, 2nd Edition a wizard who concentrates on using phantasmal force could cause the illusion to react appropriately, in Dungeons and Dragons, 3rd Edition the rules for illusions changed but that text nonetheless got left behind. Someone just didn't get the memo.
This becomes clearer when one considers that the 1st-level Sor/Wiz spell silent image [illus] (PH 279), the 2nd-level Sor/Wiz spell minor image [illus] (PH 254), the 5th-level Sor/Wiz spell persistent image [illus] (PH 260), and the 6th-level Sor/Wiz spell permanent image [illus] (PH 260) all lack that text that's included in the spell major image. None of those spells say, "The image disappears when struck by an opponent unless you cause the illusion to react appropriately" (PH 252), and none of the spells that are higher level than the spell major image inherit that text from the spell major image.
Many spells in the Player's Handbook for Dungeons and Dragons 3.X contain echoes of earlier versions of the Dungeons and Dragons game, sometimes controversially (e.g. the 4th-level Clr spell freedom of movement [abjur] (PH 233), the 8th-level Sor/Wiz spell trap the soul [conj] (PH 295-6)) but sometimes accidentally, as is the case here.
I suggest a house rule that excises that paragraph from the spell major image and that you, instead, just use the standard illusion rules (such as they are) on PH 173-4.
Illusions have a listed AC, but it is Dumb.
Illusions have a listed AC of 10 + size modifier, which is AC for unattended objects. This makes zero sense for an actively controlled illusion which is, for example, dodging and weaving. I would suggest that a reasonable houserule is to have the illusion have the AC of that which the illusion is depicting - 5 for a wall or floor, 10 +/- size modifier for an object, and use the rules I suggest below for a creature.
Beating an illusion is typically a matter of making a Will save to disbelief it after 'interacting' with it, a term that is the basis for many, many arguments. I'd say that a safe interpretation is that any time any character takes an action that directly involves the illusion (as a target, something you're walking on, walking through, trying to pick up, etc) they get to make a Will save to disbelieve.
Other than spells such as Steadfast Perception, True Seeing or Dispel Magic, or certain class features, there is no other way to beat an illusion.
On the Houserule Side, I think it is reasonable to assign an AC to a specific controlled illusion, and if the attacker successfully strikes the creature, give them a bonus on their Will save to disbelieve, or allow them to conclude it is an illusion without necessarily 'seeing through' the spell.
An AC equal to the 10 + size of the creature (Big and Little Creatures in Combat) + half the caster's caster level (CL) + the caster's casting stat mod should provide an AC that is appropriate for the caster's Challenge Rating to the illusionary monster, and allows a bit of versimilitude if the Fighter hits the creature but it's reaction is off.
It's reasonable in my opinion to allow a +2 circumstance bonus to the save, or allow someone who connects with the creature to realize it is an illusion without being able to 'see through' it, or both.
There's no check, but the opponent gets a save.
Under the rules listed for the Figment subschool of Illusion (link), figments have an AC of 10 + their size modifier. If an attacker succeeds on an attack against that AC, the wizard can simply make the illusion act like it got hit. Since the spell description doesn't say anything about needing a check for making the illusion react appropriately, the wizard can simply do it. Since the attacker is interacting with the illusion, they get a save to disbelieve.
When using Silent Image or one of its successors, you can do any manipulation of the illusion that you want within the stated limits of the spell without a check. There are no official rules for requiring checks to manipulate illusions.