I’ve cut this article and issued a correction. While there remains a very serious copyright problem on the Amazon Kindle store, the issue is complex, with game concept art being sold as stock images. So, if you see your favorite character on the cover of a book, don’t assume foul play. It’s very possible that the rights holder has given the OK.
Updated (10/09/2016 3:35 PM):
This article covers the plagiarism problem in Kindle’s now notorious erotica genre.
Updated (10/09/2016 3:30 PM):
This article gives a solid overview of the history of plagiarism and unfair use among authors on Kindle.
Updated (10/09/2016 3:25 PM):