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>');