Typically, 8" is too large an area to leave unquilted. For photos, use invisible thread or a color that matches the areas to be quilted. I like to stitch along outlines of shapes in the picture, such as around the head, or along the background or horizon lines. Follow the shapes as if they were line drawings... Just a few lines will be needed. Stay off of faces, just go around the major shapes so you have stitching approx 3"- 4"apart.
Clear as mud?