Good question. Complex answer. Please review this: https://opengameart.org/content/artificial-intelligence-assisted-artwork

Let us know if you have any remaining questions. 

> Is it okay to train AI on your assets and/or modify your assets using AI?

I don't think this is something that OGA has any jurisdiction over; only the licenses do, and the licenses grant permission for any use, even disagreeable ones (which is a core tenet of the libre philosophy since the 20th century, I'm afraid to say). Sketchfab is in a similar situation, and here's what they had to say:

> If a creator applies the NoAI tag to a model they’ve made available under a Creative Commons license, the tag may not be enforceable outside of the Sketchfab platform. This means that for CC-licensed work, the NoAI tag might not be respected because the CC license may allow use of the work by generative AI programs regardless of the tag. This is due to the way CC licenses work and is not something Sketchfab can control.
https://sketchfab.com/blogs/community/restricting-generative-ai-use-of-free-models/

Creative Commons themselves are unlikely to add a restriction in, say, version 5.0 of their licenses, given their official viewpoint is that training generative models on even wholly unlicensed data is fair use, and their recommendation for those who don't want that to happen is Asking Nicely.

I will note that, if it's decided that models are non-fair·use derivative works of their training data, the attribution requirement in almost all licenses (except CC0) will mean it's still an infringement, unless you catalog and credit every piece of data that went in (not impossible!). CC0 would still be trainable on, unless the governments of the world decided to establish an entirely new opt-in right for AI training (a la moral rights), which at least in the current US climate feels unlikely (though there have been stranger carve-outs).

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> while still claiming his game uses real artists' art for PR purpose

Hmm, "contains human assets" vs "doesn't contain generated assets"... For whatever it's worth, both Steam and itch.io expect you to disclose the existence of any of the latter. Either way, I'd imagine style mimicry would still be looked down upon and called out, even in your scenario.

I think the example you gave independent of AI usage or not but more about the license the work is in. 

Example: you can take any of my art-work, modify with any tool or even use it as is for a commercial project and fully avoid compensating me or even ask permission. 

You can even fully avoid putting my name (or nickname) in the credits, and legally that's fine because all my work is CC0. 

May not be the same with other licenses, like full copyright ('all rights reserved') or CC-BY,SA,NC.  

And about your question. Sometimes when making a game you may need to modify the arkwork you use. Eigther upscaling, doing a background removal, or making it part of another piece (like a background).  Heck, i even make style-transfers in my own art when i need.

Talking about styles. Personally, I won't complain if somebody else train a model using my art. Art-styles are not copyrighteable only the art-work is. (You can't copyrigh Anime art style as a whole. You can copyright your anime OC.) 

And I specifically resigned my rights in my own artworks, hence it's CC0.