No, sorry, I am clearly not explaining the problem properly! The imac comes with three pre-made stacks - applications, downloads and documents. The applications stack has an A on it, the downloads stack has a downward arrow, and the documents stack has a folder icon on it. The applications A is still there, but the other two icons have disappeared. The stacks appear as two plain folders., with no icons on them. When you open each stack, they behave perfectly - it is just the icons that are missing. The downloads folder in the HD clearly has the arrow, but when I drag it to the dock, the arrow disappears
Hope that is clear