To add an image to the menu, you can use a function like this:
add_action('generate_inside_navigation','generate_navigation_logo');
function generate_navigation_logo()
{
?>
<div class="site-logo">
<img src="THE URL TO YOUR IMAGE" alt="" />
</div>
<?php
}
Then add this CSS:
.main-navigation .site-logo {
float: left;
line-height: 60px; /* Adjust this to your menu item height */
margin-right: 1.5em;
}
Adding PHP: http://generatepress.com/knowledgebase/adding-php-functions/
Adding CSS: http://generatepress.com/knowledgebase/adding-css/
Hope this helps 🙂