$meta = wp_get_attachment_metadata($imageid);
if( array_key_exists("full", $meta["sizes"]) ) {
   $imagepath = wp_get_attachment_image_src($imageid,'full')
} else {
  // Fallback to the original file name
  if( array_key_exists("file", $meta) )
     $imagepath = 'wp-content/uploads/' . $meta["file"];
}