If you want to add your own custom bullete in amChart as shown in below image, just try to use below code:

graph.customBullet = "images/bullet-point.png";
graph.bulletSize = 7; // bullet image should be a rectangle (width = height)
// graph.customBulletField = "customBullet"; // this will make the graph to display custom bullet (red star)
graph.customBullet = "images/bullet-point.png";
graph.bulletSize = 7; // bullet image should be a rectangle (width = height)
// graph.customBulletField = "customBullet"; // this will make the graph to display custom bullet (red star)