Add this to your personal.ini under section [dramione.org]:
That will suppress the ribbon being used as a cover image. I'll include that in defaults.ini in the next version, too.
The ribbon will still appear in the summary, however.
In that particular story, the banner images you refer to are in the individual chapter descriptions. FFDL doesn't pick up chapter descriptions, so it doesn't automatically use any of those images. If you look, I expect you'll find that the stories that do pick up those images automatically also have them in the chapter text.