First off I want to congratulate the Wizard for coming up with such an innovative plan for stopping a dragon.
All Spell DCs will be based on your Wizard's primary spell DC
So, at base, this is 10 + Spell Level + Primary Casting Attribute Modifier (Int for the wizard). Assuming an Int bonus of +4, the save DC on Mind Fog is going to be 19.
Failing that first save is going to make the following a lot harder:
There is not a modifier for the disbelief save
The DC is going to be 10 + 3 + Casting Attribute.
But your baddie must interact with the Illusion first, before getting a save. So for Cthulhu, it would mean probably coming into contact with the illusion physically. Now a Dragon is pretty smart, and probably going to think that it's an illusion (especially if it doesn't think someone is dumb enough to summon an Outsider to stop it). It could charge Cthulhu head on if it's brash (and actually assumes it's an illusion). Though, safer ways to go about that would be to try Detect Magic and on round 3, you can determine the school of magic. Or even faster is True Seeing which will get past the illusion instantly.
So, your Wizard will not want to have Cthulhu actually interact with the Dragon, otherwise it'll trigger the save.
It should be noted that the Dragon will probably make it's spellcraft roll to identify Black Tentacles, but it may or may not assume that is being caused by the weird outsider and not some illusionist trying to save the King.
To address the Ghost Sound + Silent Image save DC
You'd save against them separately at their different DCs. Though, if you succeed on the save for Ghost Sound, I would count that as "interacting" with the Silent Image, since you will realize that the sound the illusion was making is an illusion.
Edit: As @SimonGill points out, the enemy army would most definitely freak out seeing this, and would probably not get their disbelief save...
If a creature doesn't want to be noticed casting spells then, yes, the creature must devote resources to doing exactly that, and that includes Enchantment spells. By eliminating the inexpensive material components with the feat Eschew Materials, somatic components with the feat Silent Spell, and verbal components with the feat Still Spell one can cast a spell nearly undetectably--from onlookers1 (an affected creatures still gets that tingling feeling when it succeeds on a saving throw versus a hostile spell and still has a chance to identify the spell cast on it via a Spellcraft skill check). But, yeah, it's difficult to make casting happen stealthily. And it should be.
Your question focuses on the lack of utility of Enchantment spells when the target knows such a spell's been used on it. I'd be more concerned about the increased utility of Evocation spells were they employed in this nearly-impossible-to-determine-the-source fashion.
The Spell Charm Person
The expectation is that spells charm person et al. actually are cast right out in the open--at an approaching potential enemy. And, if the enemy fails the saving throw he very well might ask, "What did you do?" if the creature is unable to identify the spell as it was cast (and most creatures can't). That's when the caster says, "Just a detect spell; don't worry about it," and, as the enemy is now you're friend, he doesn't.
However, I get the sense that your group might want to use the spell charm person against shopkeepers, city guards, buxom barmaids, and other folks in the middle of town in an effort to get free goods, to encourage a slap on the wrist instead of the gallows, to make them pay more attention to you than the muscleheaded barbarians, or whatever. In those cases, casting a spell can be problematic.
I'd urge casters to role-play these events, using minor magics like the spell prestidigitation to instantly clean the shopkeeper's store, shine the guards' shoes, or conjure a gift for the lady instead of whipping out the charm spells. Harnessing the power of the universe to get a 10% discount on swords, to avoid paying a parking ticket, or to get a lady to pay attention to you2 is... excessive.
Casting in the Campaign
I can't know the kind of campaign you're running, but in my campaigns folks are familiar with magic even though most lack ranks in the skills Spellcraft and Knowledge (arcana). Intelligent creatures in civilized areas view spellcasting the same way many contemporary societies view taking out a pistol and waving it around. You want to clear the bar? Start casting a spell. You want folks to run for cover? Start casting a spell. Spells are worse than firearms--nigh undetectable until used and can level cities.
Polite casters in my campaigns quickly learn to warn onlookers that they're casting a spell, explain what the spell is (making Bluff skill checks if necessary), and provide those who want to the opportunity to leave. Impolite casters get grappled or stabbed by folks claiming self-defense.
The campaign needs to establish guidelines for casting spells in populated areas because, yeah, spells are awesome... if you're the caster. Spells are death via a handful of bat guano, throwing horns, and yelling fireball! to everyone else.
- There doesn't seem to be Pathfinder equivalent of the Dungeons and Dragons, 3rd Edition feat Invisible Spell (Ci 61), but I have no doubt there will be. There is, however, the third-party feat Secret Spell, which, while not making the spell's effects invisible, makes stealthy casting easier.
- Depending on the lady, obviously.
Best Answer
Appearance-changing illusions are glamers not figments
Using an illusion to alter a creature's appearance is the province of spells of the illusion subschool glamer like disguise self, hallucinatory terrain, and veil. A spell like illusory wall or silent image is an illusion spell of the subschool figment, and those spells typically add information to the environment rather than change already-present information. This GM would rule that the spell silent image can't be used to change a creature's appearance, even if the caster adds to the environment a floating, stationary hood beneath which the caster stands. Seriously, illusions are complicated enough without trying to force one illusion spell's effect into a different subschool. (Instead of trying to use the silent image spell this way, this GM suggests buying a hat of disguise or using the Disguise skill: this latter is free, and others can help you… or someone else who's better at the skill Disguise than you can put a disguise on you!)
Spell effects are stationary unless otherwise indicated
The spell silent image possesses the entry Range long (400 ft. + 40 ft./level) and and the entry Effect visual figment that cannot extend beyond four 10-ft. cubes + one 10-ft. cube/level (S). The spell's range—as per Aiming a Spell—indicates that the caster
…But the effect created by the silent image spell does not say it's mobile in the same way, for example, that the sphere created by the spell flaming sphere is or that the creatures brought forth by a summon monster effect are. That is, the silent image spell says that the caster "can move the image within the limits of the size of the effect," yet that means the created image is limited to moving within the spell's shapeable area. (Again see Aiming a Spell for an explanation of a shapeable area). In short, a spell's point of origin is fixed when it's cast unless otherwise stated, and the spell silent image does not say otherwise, this despite the fact that the image can roam freely within the designated area around the spell's point of origin.
Ask the GM if locales have unique points of origin
Whether existence is composed of unchanging, stationary-in-space 5-ft. squares over which everything moves or if 5-ft. squares travel with the map that they're attached to is something for the GM to decide. (Also see this question—while for 3.5e same rules apply to Pathfinder.)
While the first ruling is elegant, the second ruling allows many spells to be cast on, for example, moving ships (although Pathfinder sometimes creates unique rules spells when they're cast on boats) and, similarly, carriages of size sufficient to have their own maps—often in such a way that makes spells comprehensible, manageable, and playable. Thus, under this second ruling—which this GM supports—, a carriage big enough to warrant its own unique map should be able to support the point of origin of a silent image spell's effect and that effect would move with that big carriage, but a carriage that's too little to warrant its own map will see it leave a silent image effect in its dust.