I would start with a DMCA notice to the publisher of the book.  A cover letter can be included , be polite, no threats, but you may choose to offer them the picture at normal commercial rates.

 

You need adequate evidence that the pic is yours , so a website that shows when the pic was uploaded would do.

 

Realize that anything said to anyone may be used if the matter goes to court. Also realize that an unregistered copyright is not eligable for punative damages, only real damages which could be hard to show if you dont have previous sold work.

 

At least get your name on the credits for the cover if they continue with that image.

 

some lawyers will do a letter for about $50.  A letter from Aron, Benson and Swartz, Attorneys at Law may get quicker results than a letter from Billy Bob of Podunk, Arkansas.

 

• no offense to any Billy Bobs or residents of Podumk or Arkansas.