// cache images
   imgBlue = new Image(24,18);
   imgBlue.src = "images/bullet_blue.gif";
   imgGreen = new Image(24,18);
   imgGreen.src = "images/bullet_green.gif";
   imgGrey = new Image(24,18);
   imgGrey.src = "images/bullet_grey.gif";
   imgOlive = new Image(24,18);
   imgOlive.src = "images/bullet_olive.gif";
   imgOrange = new Image(24,18);
   imgOrange.src = "images/bullet_orange.gif";
   imgPurple = new Image(24,18);
   imgPurple.src = "images/bullet_purple.gif";
   imgRed = new Image(24,18);
   imgRed.src = "images/bullet_red.gif";
   imgTurquoise = new Image(24,18);
   imgTurquoise.src = "images/bullet_turquoise.gif";
   imgYellow = new Image(24,18);
   imgYellow.src = "images/bullet_yellow.gif";