There's nothing necessarily wrong with your fill stitch. It just depends on the look you are going for. If it's filled in but textured, just keep going like you have until it's filled. If it's filled in but flat, then try a satin stitch.
You could always recreate it by actually drawing it or printing it onto the fabric. then embellish over the top with your stitches. Using different numbers of strands for the floss will help with the details too.