Sparky

In menu item settings under Link Type Options you have "Link CSS Style" property. You can put sometihng like " mybutton" (note first char is blank).

Then add css class like following to tempalte_css.css:

.mybutton{
display : block;
background : url(...)...
...
}

Menu successfully change the background but how about the sub menu button and panel?

For sub-menu item use same approach. For drop-down panel you cant control that individually, you can add general style like:

UL.menu.nav UL{
...
}