Path_to_theme function problem in drupal6

I try to use path_to_theme function in drupal6, to show the image in my theme, but wen i use it always get strang problem, some page display wrong, like forum front page display empty page, but there have some containers and forums, when an enable the path_to_theme, use 'sites/all/themes/mytheme/" instead of it, the forum fornt page works, i don't know why get this error, it's a bug for drupal6?
i hope some body can help me about this.