I guess I found the code used for the post gallery format, in image.php :
<?php
/**
* Grab the IDs of all the image attachments in a gallery so we can get the URL of the next adjacent image in a gallery,
* or the first image (if we're looking at the last image in a gallery), or, in a gallery of one, just the link to that image file
*/
$attachments = array_values( get_children( array( 'post_parent' => $post->post_parent, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID' ) ) );
foreach ( $attachments as $k => $attachment ) {
if ( $attachment->ID == $post->ID )
break;
}
$k++;
// If there is more than 1 attachment in a gallery
if ( count( $attachments ) > 1 ) {
if ( isset( $attachments[ $k ] ) )
// get the URL of the next image attachment
$next_attachment_url = get_attachment_link( $attachments[ $k ]->ID );
else
// or get the URL of the first image attachment
$next_attachment_url = get_attachment_link( $attachments[ 0 ]->ID );
} else {
// or, if there's only 1 image, get the URL of the image
$next_attachment_url = wp_get_attachment_url();
}
?>
<a href="<?php echo $next_attachment_url; ?>" title="<?php echo esc_attr( get_the_title() ); ?>" rel="attachment"><?php
$attachment_size = apply_filters( 'spun_attachment_size', array( 1200, 1200 ) ); // Filterable image size.
echo wp_get_attachment_image( $post->ID, $attachment_size );
?></a>
If the href attribute directly points to the image URL instead of the attachment ID, it will work. But I have no idea how to modify the code ?
Can somebody advice on this ?
Thanks.