If I have a standard Skin.CSS file for my skin, and I have something like this:

#menu a {
   ...
   background: url("button.gif") top left no-repeat;
   }
#menu a:hover {
   ...
   background-image: url("button-over.gif");
   }
#menu a:active {
   ...
   background-image: url("button-active.gif");
   }

When URL's are referenced without a full path, i.e. www.domain.com/portals/1/button-over.gif, WHERE is the page going try and pull the file from by default?  Is DNN going to pull button.gif from the DNN root directory, the Portal ID# directory, the Skin's folder root directory, or what?

The question is, in essence, where do I put the images I want to use for buttons, etc., if I'm going to just name the file in the url("button.gif"); part of my CSS file?