It would be beneficial if you had a way to provide additional information (whether downloadable copy of the skin, live demo, or otherwise).  In general, however, you can use an image as a background merely by using the "background-image" css tag in the skin.css.  As long as the image is in the same folder (or sub-folder) of your skin folder, it would be something like: background-image: url(someimage.jpg); (or somefolder/someimage.jpg)

Something to note is you *might* have CSS elsewhere in the portal (portal.css, default.css, module.css, etc) that is overriding whatever/wherever you are trying to place that image -- this, if you are using a "base" dnn css class.  Using FireBug in FireFox will help you peek at the CSS and see what class combination ends up being the winner and then you can adjust your CSS if necessary.