Why is there a dragon on the cover of the Ven Polypheme books?

I’m reading The floating island right now (by the way it’s so amazingly good!!) and there aren’t any dragons in the book that I know of and the other books in the series also have a dragon on the cover. Is there an actual reason for this, or is it just on their? And Ven isn’t a dragon, he’s a Nain.