A good design to start with learning free motion is a gentle curve.
Simply beautiful! It reminded me of a creek with varying shades of sand and soil and pebbles. I think gentle curving lines going horizontally across would look like the water ripples going over the creek bed. This shape is a beginner free-motion quilting shape...think cursive lower case m but elongated to be almost flat.
You can audition quilting designs by printing out a photo of your quilt top and putting a clear plastic sheet (I use page savers) over the photo. Next take a dry erase pen and audition the designs you have in mind. Practice on a quilt sandwich of the same fabrics, batting, and thread that you will use in your quilt to audition thread color and check tension before you start on you quilt. Most of all, have fun and show us what you decide.