How to Display image from Menu Image in Walker Dropdown Select Menu

It’s stored in metadata, there are four meta fields: _thumbnail_id and _thumbnail_hover_id – store attachment ID, _menu_item_image_size – store selected image size, _menu_item_image_title_position – store label position, but you can get stored values from this properties:
$item->thumbnail_id, $item->thumbnail_hover_id, $item->image_size, $item->title_position. Check the menu_image_nav_menu_item_filter() function in menu-image.php