Since, once someone downloads graphic assets, they basically own them.
Software piracy and copyright. They do not own them after a refund. They could always just steal your assets from other games. Why the hassle of demanding refund and still use them. You know they have them and could watch their release and screw them majorly if they use the refunded assets.
And since there is no drm on Itch, refunds are on a case by case basis. As contrasted to Steam with their ability to remove the game from your account, so they can offer a no questions asked refund policy.
If your assets are not what you promised, a no-refund-policy will not protect you from refunds.
Legally, you will ony be able to implement a refund policy that is better for the customer, not worse, than the platform's rules. The case by case dispute resolve is already the best you can have:
You may be eligible for a refund if the product does not work or does not represent what was advertised.
Is that not what you wanted to have as an exception?