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add_custom_image_size_media_window.php

<?php

/****
* New image size
* @gregoirenoyelle
****/

// Create a new image size
add_image_size('Vign Haut de Page', 266, 168, TRUE);


// add new image size in insert window
add_filter( 'image_size_names_choose', 'gn_image_sizes_choose' );
function gn_image_sizes_choose( $sizes ) {
	$custom_sizes = array(
		'Vign Haut de Page' => 'Vign Haut de Page' // add other size in array 
	);
	return array_merge( $sizes, $custom_sizes );
}