inserting and scaling images using wp_insert_attachment

I had the hardest time trying to figure out a similar issue to that. I was able to work around it by using the built in function media_handle_upload(). This wouldn’t work in your case, as it only handles images uploaded in a $_FILES post. But you could take a look at the source of that function to see how it generates the intermediate image sizes.

The function I ended up with, after tearing my hair out trying to make wp_insert_attachment work for me, was just real simply:

function insert_attachment($file_handler,$post_id,$setthumb='false') {

		require_once(ABSPATH . "wp-admin" . '/includes/image.php');
		require_once(ABSPATH . "wp-admin" . '/includes/file.php');
		require_once(ABSPATH . "wp-admin" . '/includes/media.php');

		$attach_id = media_handle_upload( $file_handler, $post_id );

		if ($setthumb) update_post_meta($post_id,'_thumbnail_id',$attach_id);
		return $attach_id;
	}

where $file_handler refers to an object in the $_FILES superglobal.

Hope this helps someone.

Correction: the $file_handler parameter above actually refers to a key in the $_FILES array (i.e. a file upload field from a submitted form), not the object itself.