I opened up the skin.css file for the DNN-gray skin and set the content pane style to this:
.contentpane {
width: 100%;
background-color: transparent;
padding-left: 6px;
padding-right: 4px;
padding-top: 6px;
background-image: url(/Portals/5/red_leaf_fd_sm.jpg);
background-repeat: no-repeat;
}
Then I copied the red_leaf_fd_sm.jpg file into /Portals/5 in my test site. It worked just fine.
If you place the file in the skin directory, then you can specifiy url(red_leaf_fd_sm.jpg) in the style definition. This is how Lee does things in the Red Leaf Skin tutorial, and I think that it makes sense to keep everything together, assuming that you think of the background image as part of the skin rather than as part of the site.
Take a look at
http://jncraig.com/test/Gray/tabid/...fault.aspx to see this in action.