Gee, that's a familiar looking book form!
I do exactly you've asking for myself.
First, for existing books, Edit Metadata--with one book, you have to choose the format (epub, I assume), and click the button to the upper left of the formats list that has a book cover icon. For multiple books Edit Metadata and toward the bottom under 'Change Cover' choose 'Set from ebook file(s)', the OK or Apply.
In FFDL config on the Basic tab, turn on the option 'Update Calibre Cover when Updating Metadata?' (2nd under Updating Calibre Options heading), then FFDL will do it for you when ever you add or update a book from FFDL.