doc: Reorganise images and update links

Change-Id: I679d1499376a524bef1cfc33df995b0a719b5ac8
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/components/romlib-design.rst b/docs/components/romlib-design.rst
index ab39723..a70ed17 100644
--- a/docs/components/romlib-design.rst
+++ b/docs/components/romlib-design.rst
@@ -23,7 +23,7 @@
 Index file
 ~~~~~~~~~~
 
-.. image:: diagrams/romlib_design.png
+.. image:: ../resources/diagrams/romlib_design.png
     :width: 600
 
 Library at ROM is described by an index file with the list of functions to be
@@ -54,7 +54,7 @@
 Wrapper functions
 ~~~~~~~~~~~~~~~~~
 
-.. image:: diagrams/romlib_wrapper.png
+.. image:: ../resources/diagrams/romlib_wrapper.png
     :width: 600
 
 When invoking a function of the "library at ROM", the calling sequence is as