[FIX] Use the picture widget instead of image, because it's a readonly field

lp bug: https://launchpad.net/bugs/324830 fixed

bzr revid: stephane@tinyerp.com-20090203161300-95yds9v0k0js4tyt

bin/addons/base/ir/ir.xml

@@ -583,7 +583,7 @@
                         <separator string="Preview" colspan="4"/>
                         <field
                             name="preview"
-                            widget="image" 
+                            widget="picture" 
                             readonly="1" 
                             nolabel="1" 
                             colspan="4"