From one of my tools:
message="Does some stuff."
tip="Does some stuff"
As you can see the image has an extension and the XML is properly closed.. your XML should read like:
<Button id="GridViewMenu" caption="Element View" image="Images\gridview.png" isRootMenu="false"/>
<Button refID="your_namespace_GridViewMenu" />
Where the properties of the tool are declared in the
<Commands> enclosure an then the tool is added to the menu later on by using its fully qualified name.
I am assuming that a menu follows the same layout as a toolbar. All my existing menus are 'old style' like this and not generated via XML.