Change name of gif file and add it to SVN

git-svn-id: http://root.cern.ch/svn/root/trunk@23721 27541ba8-7e3a-0410-8455-c3a389f83636

gui/doc/v520/index.html

@@ -62,5 +62,5 @@ The context menu of ROOT classes can be created with hierarchical sub-menus, whi
 
  EPaletteType fPalette; //*OPTION={GetMethod="GetPalette";SetMethod="SetPalette";Items=(PrettyPalette="PrettyPalette",SchlenkPalette="Schlenk",pal3="Pal3",pal4="Pal4")}*</pre>
 
-<p><img alt="Hierarchical context menus." src="pictures519/contextmenu.gif">
+<p><img alt="Hierarchical context menus." src="gui_contextmenu.gif">
 <p>