Hi there!
To add a new menu title, you need to create 3 new images, because this menu changes two images when you roll over an item. You will need to create a "mouseover" graphic, a "mouseout" graphic, and a digit graphic (e.g. the number 7).
Say your new menu item is called "newitem". Then add the following JavaScript code to the page:
newitemroll = new Image(106, 68);
newitemroll.src = "images/menu/roll_7.gif";
newitem_on = new Image ( 134, 94 );
newitem_on.src = "images/menu/newitem_on.gif";
newitem_off = new Image ( 134, 94 );
newitem_off.src = "images/menu/newitem_off.gif";
Then in the body of the page itself, you'd add the following code to produce the new menu item on the page:
<td>
<a href="newitem.html"
onmouseout="button_off('newitem');
blank.src = blank2.src;
return true"
onmouseover="button_on('newitem');
blank.src = newitemroll.src;
return true">
<img src="images/menu/newitem_off.gif" border="0" name="newitem"></a></td>
To answer your second question: The numbers in ( ) refer to the width and height of the image (in pixels).
Hope that helps!
Best wishes,
Matt

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Matt Doyle, Elated
Second Edition of my jQuery Mobile book out now! Learn to build mobile web apps. Free sample chapter: http://store.elated.com/