"All image tokens are treated as hero figures that roll a grey defense die."

What does that mean for skills that target heroes?

i.e. the "Defend" skill used by the Knight class ("When a Hero adjacent to you is targeted by...")...

Hello TamTiTam and welcome to the Forum.

Shadow of Nerekhall Image Tokens - the rules do say "Each image token is treated as a hero figure with the Conjurer’s attributes and 1 gray defense die."

Since the Image is treated as a hero figure, the Image may be targeted and affected by monster attacks, hero abilities, and Overlord cards that target a hero. (actually, this sentence is from the Familiars Treated as Figures section, but I can't see any reason why it wouldn't apply here) So when the Image is attacked, the Knight can use his Defend ability to protect the Image. If anyone disagrees with this, I'm sure we'll hear from them shortly .

Absolutely correct- worth noting that image tokens are not familiars, but those rules (for what can affect them) definitely apply. While playing last night, we discovered that the spiritspeaker's "cloud of mist" goes very nicely with the conjurer using image tokens for defense, since monsters are much less likely to eliminate them when they can't be hit easily.

It is also worth noting that most conditions do nothing to image tokens since they don't activate- they can still receive conditions, but most have no effect. For example, poison, disease, bleed, burn, etc will never cause them to need to test or take damage. They don't have actions or suffer fatigue to gain movement, so stun, curse, and immobilize also do not affect them. Weakened certainly affects the images, but doom doesn't (since they are removed if they suffer even 1 damage.)

Absolutely correct- worth noting that image tokens are not familiars, but those rules (for what can affect them) definitely apply. While playing last night, we discovered that the spiritspeaker's "cloud of mist" goes very nicely with the conjurer using image tokens for defense, since monsters are much less likely to eliminate them when they can't be hit easily.

It is also worth noting that most conditions do nothing to image tokens since they don't activate- they can still receive conditions, but most have no effect. For example, poison, disease, bleed, burn, etc will never cause them to need to test or take damage. They don't have actions or suffer fatigue to gain movement, so stun, curse, and immobilize also do not affect them. Weakened certainly affects the images, but doom doesn't (since they are removed if they suffer even 1 damage.)

but images only have one HP, so to inflict the condition would probably destroy the image, unless it's an OL card or monster than inflicts the condition without inflicting damage.

Indeed- it is difficult for image tokens to get conditions, but it can happen.

It also should be noted that Plot Cards from the Lieutenant Packs cannot target allies, familiars, or other figures treated as heroes (from the last page of the FAQ).