To show these images you must have created a filter to do so as described either here https://www.advancedcustomfields.com/resources/adding-fields-menu-items/ or here https://www.advancedcustomfields.com/resources/adding-fields-menus/. In order to add the alt tag you will need to get it and show it in your filter, this isn’t something that WP automatically does. Check out this explanation https://wordpress.stackexchange.com/questions/193196/how-to-get-image-title-alt-attribute
