касательно wp_insert_post

Читайте описание к этой функции http://codex.wordpress.org/Function_Reference/wp_insert_attachment

This function inserts an attachment into the media library. The function should be used in conjunction with wp_update_attachment_metadata() and wp_generate_attachment_metadata(). It returns the ID of the entry created in the wp_posts table.

— Используйте её в связке с указанными функциями, или используйте обёртку http://codex.wordpress.org/Function_Reference/media_handle_upload над этими низкоуровневыми функциями

This function is part of the low-level API used by WordPress for handling attachments. To perform the entire attachment upload and insertion process at once, you will want to use media_handle_upload() instead in most cases.