once you have the actual picture I would do a to scale drawing of the horse or take a copy of the picture to Staples and have it blown up to the actual size the client wants on the quilt. If needed the picture can be quartered to get the enlargement you need. You would then have an accurate working pattern to transfer to the background fabric. I would use a number of different fabric colors in order to represent the actual lights, medium and dark tones of the horse. If only one fabric is used the result will be flat with no dimensional qualities at all. You may be able to find dark batiks in browns and blacks that will work well as ..some fabrics that would seem NOT to work will add a great touch such as dark burgundy just as it works in oils and pastels. Cathy Wiggins does some fabulous photo to fabrics quilts that are incredible. Here is a link to her webpage:
http://www.cathywiggins.com/how_its_done.htm
This will be a huge time investment by you, be sure you are compensated fairly for the job. Be sure and post a picture when it's completed!! Good luck.