It's not that simple.
Since you're talking about real porn images, I'll aim towards US law.
Firstly, "Fair Use" is an affirmative defense, not a legal right. It is basically a "Yes, I'm guilty... but..." defense. As such, it would not stop someone taking you to court - since it can only be argued in court. The usual strategy is to threaten court action, but offer a relatively small settlement fee.
You could think that if you took two images and photoshop'd them together, both images will have been transformed and therefore covered by fair use. Except companies and people have argued that you haven't actually changed the nature of the photo/image. It gets really messy and the only people who are guaranteed to win are the lawyers. (Unless you are
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).
Also actual transformation is only 1 of
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. The other parts include whether your use of the image usurps part of the market for the original, whether you are using the new image for profit, whether you are using the image for criticism/parody/news reporting and generally how much of the original work you've "borrowed".
Yeah, but mine's a game and their's is a video.... except both are visual adult stories told for sexual gratification.
... as I say, it gets messy.
Which is to say, yes, you would probably be on the wrong side of copyright law.
Now the good news... Nobody cares (yet)...
There are currently
142 HTML games here on F95 alone that use real life porn images. I would imagine this is only the tip of the iceberg, as other sites are more geared up towards HTML games.
For someone to get worried about it... someone has to notice the copyright infringement. Porn games are still only a tiny pool in an ocean of copyright infringement.