You wont be able to accomplish this in the wizard, you will need to modify the menu items after the menu is created by adding inline styles:
<li><a href="#" style="width:150px;">Sub Item 1</a></li>
As for the link text, it is best not to have the link text in the image, this is for standards compliance with screen readers used by the disabled. If the text is in the image, the navigation will not be usable by such devices. If you must, though, you would use the same technique of adding an inline style, but make sure to use a name attribute for the a tag:
<li><a href="#" style="width:150px; background-image:url(menu1.jpg);" name="Sub Item 1"></a></li>