Polite contact is always your best first move!

Contact the publisher explaining that you are the owner of the work which has been used without your permission. Explain that they will either need to pay you compensation for the use of your work, or remove it and destroy all copies. Keep it professional and remember to stay professional whatever their response.

They may be a small publisher, but as a publisher they should be well aware of intellectual property rights. It's quite possible that they may not be directly responsible if they've been supplied with the image by someone else (and may be fairly horrified to discover this!), but in that case you'll still be dealing with them and it's up to them to sort it out themselves.