How to add custom label or texts in featured image

You can do this by calling a javascript file for admin.

Create a javascript file in js folder in you current theme called admin.js. inside the file add this code:

jQuery(document).ready(function($) {
    $('#postimagediv .inside').prepend('<p>Recommended image size 300*300px</p>');
});

Now in functions.php in your current theme folder add this code to call the javascript file exclusive for admin backend.

add_action('admin_init', 'custom_admin_js');
function custom_admin_js() {
    wp_register_script( 'admin-js', get_stylesheet_directory_uri() . '/js/admin.js' );
    wp_enqueue_script( 'admin-js' );
}

For different taxonomy blocks/sections you can use their identifier and in html their ID will end with div. For example if the taxonomy identifier is "some_taxonomy_identifier" the html ID will be "some_taxonomy_identifierdiv". so in js will be like this:

$('#some_taxonomy_identifierdiv .inside').prepend('<p>Some recommended text</p>');