Image Sizes

Register image thumbnails:

function images_setup() {    
    add_image_size( 'gallery-thumb', 805, 434, true);
}    
add_action('after_setup_theme', 'images_setup');

Backend editor dropdown thumbnail select:

function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'gallery-thumb' => __('Gallery Main'),
        ) );
    }
add_filter( 'image_size_names_choose', 'my_custom_sizes' );