It's wrong for a writer who isn't an artist to use a computerized art-generation tool to create her own book covers, because it steals other peoples' work, but it is perfectly acceptable to write or draw fan-fiction of someone else's copyrighted creations, and publish that stuff.
So let me get this straight...
It's wrong for a writer who isn't an artist to use a computerized art-generation tool to create her own book covers, because it steals other peoples' work, but it is perfectly acceptable to write or draw fan-fiction of someone else's copyrighted creations, and publish that stuff.