Ok, keep the changes you've made thus far, but also move the onmouseover events from your main menu items' A element to inner IMG instead:
Code:
<a href="http://www.milorihomes.com/customer/index.php" onclick="return clickreturnvalue()"><img src="client_files/nav_customer.gif" alt="Customer Care" class="navItem" width="132" height="90" onmouseover="dropdownmenu(this, event, 'customer')"></a>
<a href="http://www.milorihomes.com/blog/index.php"><img src="client_files/nav_blog.gif" alt="Blog" class="navItem" width="53" height="90"></a>
<a href="http://www.milorihomes.com/development/index.php" onclick="return clickreturnvalue()" ><img src="client_files/nav_dev.gif" alt="Development Projects" class="navItem" width="186" height="90" onmouseover="dropdownmenu(this, event, 'development')"></a>
<a href="http://www.milorihomes.com/services/index.php" onclick="return clickreturnvalue()" ><img src="client_files/nav_services.gif" alt="Services" class="navItem" width="83" height="90" onmouseover="dropdownmenu(this, event, 'services')"></a>
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