OK a number of small changes here.

Firstly, I'm replacing all use of old-style global variables like
$HTTY_REFERER with their new-style equivalent $_SERVER["HTTP_REFERER"]
Also using $_POST instead $HTTP_POST_VARS  etc

Secondly, if gdversion == 0 (ie GD is not installed) then:

 - users are not even allowed to upload new images
 - graphs now just print a message instead of failing.

this allows Moodle to still be used even if GD is not present

admin/lang.php

@@ -130,8 +130,8 @@
 
     } else if ($mode == "compare") {
 
-        if (isset($HTTP_POST_VARS['file'])){   // Save a file
-            $newstrings = $HTTP_POST_VARS;
+        if (isset($_POST['file'])){   // Save a file
+            $newstrings = $_POST;
             $file = $newstrings['file'];
             unset($newstrings['file']);
             if (lang_save_file($langdir, $file, $newstrings)) {