Why is the background image invalid under ie6_html/css_WEB-ITnose

Use png format image as background image
background-repeat:repeat-y;
width:713px;
float:left;
padding-bottom:30px;
overflow: hidden;
position:relative;
background-image:url("bjt1.png");

Why is it invalid to create a web page background in IE6? It cannot be displayed.

Reply to discussion (solution)

ie6 does not support png format
Reference
http://www.cnblogs.com/yuzhongwusan/archive/2009/04 /17/1438119.html

ie6 does not support png