you would need to edit the webassist/plugins/main_menu/menu.php file and add an inline style for the background image, for example:
change:
<li class="top"><span><a href="../../../index.php">Home</a></span></li>
to:
<li class="top"><span><a href="../../../index.php" style="background-image: url('plugins/path/to/image.jpg');">Home</a></span></li>
the dimensions will change from theme to theme, 90 X 35 is a good starting point though