Add item to top of menu using a filter in functions.php

Use $items="Blah".$items instead of $items .= 'Blah'

add_filter( 'wp_nav_menu_items', 'add_logo_to_menu_item', 10, 2 );

function add_logo_to_menu_item ( $items, $args ) {

    if ($args->menu == 'main-menu') {
        $items="<li><a href="#"><img src="PATH_TO_IMAGE"></a></li>" . $items;
    }

    return $items;

}