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function register_image_sizes() {
    add_theme_support('post-thumbnails');
    add_image_size('home-slider', 1000, 500, true);
    add_image_size('gallery-image', 350, 277, true);
}

add_action('init', 'register_image_sizes');

function add_custom_sizes( $imageSizes ) {
  $my_sizes = array(
        'home-slider' => 'Home Slider (1000x500)',
        'gallery-image' => 'Gallery Image (350x277)',
    );
    return array_merge( $imageSizes, $my_sizes );
}
add_filter( 'image_size_names_choose', 'add_custom_sizes' );