Following is code i use to insert images.
[code]
foreach  ( $extra_images as $extra_image )
  {
// Download file to temp location
$tmp2  =  download_url( $extra_image )  ;

// fix file name for query strings

preg_match('/[^\?]+\.(jpg|JPG|jpe|JPE|jpeg|JPEG|gif|GIF|png|PNG)/', $extra_image, $matches)  ;

$file_array2['name'] = basename( $matches[0] )  ;

$file_array2['tmp_name']  =  $tmp2  ;

$imgID = media_handle_sideload( $file_array2 , $post_id , 'desc' );

$imagearray[]  =  $imgID ;

}

$imgids  =  implode(' , ', $imagearray ) ;

$result  =  update_post_meta( $post_id, '_product_image_gallery', $imgids ) ;
[/code]