The main thing that you want to avoid is giving the client the 'after' picture unless they have paid for it, handsomely. Also, if they want to just buy the picture, you should point out that the picture does not depict scale so installing from it may not lead to the desired results. The object of this whole exercise is to sell the installation. Using a rebate of the design fee is often a good indicator of how serious they are about hiring the work to be done. If they comment that they can buy the plants cheaper elsewhere, maybe you are too busy to take on their project.