Strange. The menu has StaticEnableDefaultPopOutImage="false" which should stop any image being rendered at all!
I notice you have a DropDownList to select the theme on the page as well. Can you see in any of the .skin files if this is turned back on? There would be a line looking something like:
Code:
<asp:Menu StaticEnableDefaultPopOutImage="True" />
In whichever case, you should still get an image if this is enabled, as the default is to use an arrow image built into the .NET dlls. This can be overriden by setting StaticPopOutImageUrl in the <asp:Menu> tag, or PopOutImageUrl within an item.
Phil