// Show image dimensions in media library overview
function wh_column( $cols ) {
        $cols["dimensions"] = "Dimensions (w, h)";
        return $cols;
}
add_filter( 'manage_media_columns', 'wh_column' );
function wh_value( $column_name, $id ) {
    if ( $column_name == "dimensions" ):
    $meta = wp_get_attachment_metadata($id);
           if(isset($meta['width']))
           echo $meta['width'].' x '.$meta['height'];
    endif;
}
add_action( 'manage_media_custom_column', 'wh_value', 11, 3 );