Please check to see if you're running in safe mode.

An easy way to do this is to create a file called phpinfo.php. It should contain these 3 lines:

<?php
phpinfo();
?>

Put it in your forums directory and point your web browser at it. If you search for 'safe_mode', you should find 2 lines. safe_mode and safe_mode_exec_dir. If safe_mode is on, then safe_mode_exec_dir HAS to contain your a copy of convert. If you don't have shell access/admin access to your server, I doubt you will be able to make this change.