can't remember if there's a drawStar code like drawOval.
GreenfootImage image = new GreenfootImage(15, 19);
int[] xs = new int[] { 0, 9, 18};
int[] ys = new int[] { 14, 0, 14);
image.setColor(java.awt.Color.blue);
image.fillPolygon(xs, ys, xs.length);