Hello,
It may seem like a silly question, but have you tried adding the image details to the submenu class?
.main_dnnmenu_submenu {
background-color: #cad5ea;
z-index: 1000;
cursor: pointer;
cursor: hand;
filter:progid
XImageTransform.Microsoft.Shadow(color='#696969', Direction=135, Strength=3);
padding: 1px 1px 1px 1px; /*explicitly set padding for sub-menus */
background-image: url(button_off.gif);
}