There are lots of complexities here, but the first questions to look at (from an English law perspective) are: does the EULA (aka licence agreement) for the game allow you to create and publish/distribute artwork in this way? Does the EULA allow you to earn money in this way?
If the EULA allows for this use, and does not prohibit you from earning money in this way, then it is possible (but not 100% certain) that you are not in breach of contract or infringing the copyright or other IP rights.
If, on the other hand, the EULA does not allow you to publish/distribute artwork in this way, or if it prohibits this type of commercial exploitation, then it is likely (again not 100% certain) that you are in breach of the EULA and/or infringing copyright or other IP rights.
In either case, I think you will need to properly consult a specialist IP lawyer to get a reasonably certain answer.