The cover of the book, drawn by Johann Wessels, features the Ribbajack himself, a monster created by one's imagination out of pure hatred for someone else. According to legend, once the mental image of the Ribbajack is clear, it will come alive at midnight and destroy your enemy. A typical Ribbajack is described in the short story as having "the body of a crocodile, three eyes, [and] long poison teeth", however you can make your Ribbajack look as evil as you wish, because after all, it is your creation.
I recently communicated with Mr. Wessels, who agreed to share his conceptual art for the Ribbajack cover. It was also neat to learn that his sons are actually Redwall fans too. If you're interested in more on what Mr. Wessels is up to, be sure to check out his blog, here.