Realistically, no one is going to bug you about copyright for these games. Personally, I've never seen or heard of any instance where an artist complained that their work was used without permission.
However, you are absolutely 100% wrong if you think that just because you're doing something for free, copyright laws don't apply to you.
It's also complete bs to claim that you're advertising his work for free by using it in your story, and therefore that makes it an equivalent exchange.
Copyright is not contingent upon profit or benefit. You may not use another person's creation without their permission. Period.
What you are describing are the conditions for using work under a Creative Commons license. Most work is not Creative Commons, and it will specifically state if it is. (it's the cc in a circle logo)
Creative Commons licenses use one or more of the following rules:
Attribution: You must credit the original creator
Non-Commercial: You cannot profit from a work utilizing the content
No Derivative Works: You may not change or alter the content in any way
Share Alike: You may change, alter, or adapt the content, but any work created must also be distributed with a Share Alike license. (In other words, you can't put a traditional copyright on your work if it uses any part of a Share Alike licensed work.)
If the creation in question is not Creative Commons (or public domain) you may not use it without the owner's permission. Doesn't matter if you credit the owner, doesn't matter if your work is "a labour of love," doesn't matter if you think you're doing the owner a favor by providing a wider audience for his work. You cannot use another person's creation without their permission.