I'd suggest doing some reading first:
www.ipo.gov.uk
If you've not done anything already to protect your work, your easiest route may be to invoice him for the use of your picture, since the legislation exists to protect these.
Eventually with all of these things, it comes down to how much time/effort you are prepared to put into the pursuit of it vs how much recompense/benefit you will see.
Good luck and really do have a read on the IPO site, this is a good starting spot:
http://www.ipo.gov.uk/d-basicfacts.pdf