ul{
 /* 
  list-style: [list-style-type] [list-style-position] [list-style-image];
 */
  list-style:square inside url('custom-bullet.png');
}

/* is the same as */
ul{
 list-style-type:square;
 list-style-position:inside;
 list-style-image:url('custom-bullet.png');
}