Magento which file have code of navigation menu bar?

Question

i want to edit the magento navigation menu bar code but i don't know that the which file have a menu bar code please help how can edit that file and change please help

i want to edit menu magento provide menu like
<ul>
    <li><a href=''>Home</a></li>
    <li><a href=''>Contact Us</a></li>
    <li><a href=''>Abouty Us</a></li>

</ul>

i want to edit it like

<ul>
    <li><img src=''/><a href=''>Home</a></li>
    <li><img src=''/><a href=''>Contact Us</a></li>
    <li><img src=''/><a href=''>Abouty Us</a></li>

</ul>

i want to add image into menu and create advance menu