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<?php

add_image_size( 'crop-631x300-center-top', 631, 300, array( 'center', 'center' ) );

/**
 * Filter Yoast WordPress SEO plugin Open Graph image size
 *
 * Default: medium
 * 
 */
add_filter( 'wpseo_opengraph_image_size', 'ac_wpseo_image_size', 10, 1 );

function ac_wpseo_image_size( $string ) {
	return 'crop-631x300-center-top';
}  

?>