I do not see a menu that has sub-items, and therefore don't see any arrows...  where are the arrows you are referring to?

usearrows should work.  I would need to see your skin to see what is wrong.  I noticed that you are not specifying any css specific to the root menu items, which is why your attempts to style them specifically have failed.  As your skin stands now, you have the option to do the following, but it will hide all IMG within your menu. 

.MainMenu_MenuItem IMG {
display: none;
}

The correct answer though is to change your skin with the usearrows property.  If you want to email your skin to me to have a look, feel free.