diff --git a/reference/image/book.xml b/reference/image/book.xml
index e0bab94341..d2f6fe4aca 100644
--- a/reference/image/book.xml
+++ b/reference/image/book.xml
@@ -47,6 +47,13 @@
       The <function>getimagesize</function> function does not require the GD extension.
     </simpara>
    </note>
+   <caution>
+    <simpara>
+     While the bundled version of the GD library uses the Zend memory manager to
+     allocate memory, system versions do not, so that <link
+     linkend="ini.memory-limit">memory_limit</link> does not apply. 
+    </simpara>
+   </caution>
   </para>
 
   <para>
diff --git a/reference/image/ini.xml b/reference/image/ini.xml
index f5bccca0b3..4f33d81248 100644
--- a/reference/image/ini.xml
+++ b/reference/image/ini.xml
@@ -74,7 +74,8 @@
  <warning>
   <para>
    Image functions are very memory intensive. Be sure to set <link
-   linkend="ini.memory-limit">memory_limit</link> high enough.
+   linkend="ini.memory-limit">memory_limit</link> high enough, if you are using
+   the bundled version of the GD library.
   </para>
  </warning>
 </section>