[support] new installation of Drupal 5 not finding gd

Hi, I just installed a brand new Drupal 5, on a Debian Sarge box. The admin script is reporting "The GD library for PHP is missing or outdated" (everything else reports ok). The thing is, I think I have gd installed properly and as up to date as the current package library provides (apt-get upgrade php4-gd reports that I'm up to date). So... any thoughts or suggestions? - do I need a later version of gd? - is it perhaps something in the search path (pointers or diagnostic suggestions much appreciated) - what doesn't work if I don't have gd properly installed? Thanks very much. Miles