In my custom theme I created a custom header section :

 add_theme_support( 'custom-header', array(
    'video' => true,
    'flex-width' => true,
    'flex-height' => true,
    'width' => 1920,
    'height' => 677,
   ) );

but I want to add some additional info in the admin area where you chose the images to upload, and if possible to change the default text that now is: "Click “Add new image” to upload an image... etc"

Thank you.