Well, there are various ways to thank the artist who made your novel's cover for you. I think the most basic would probably be posting a Thank You message in your wall, or simply a message giving the artist some credit. That way, your subscribers can take note of it, since I think there would be notification sent to them. Or you could just smack the credits into the summary or afterword in the series, just like what @Ordinaryaverageguy said.
