The contract can be very simple: it can simply state that the customer gives you permission to display pictures of the work you have done on their property on your website and otherwise use it for advertising purposes. For a contract to be binding, however, there must be "consideration," or something that each party receives; you are receiving the right to use the pictures, so the customer must get something, too. One possibility is to give them some small discount or some additional service as compensation for allowing you to use the pictures. The contract should state what they are getting.