Hi,
As you have noticed, the SearchResults.aspx page will use the site default skin. There is no way to tell it (or any other system page) to use something else.
In general, I would recommend a more robust method of creating the columns than using different skins with different background images. mojoPortal automatically adds columns based on where content is located (left, center, right panes). This makes the page more dynamic but can add a bit of complexity to the skin.
If your background image being referenced through CSS and isn't using an IMG element in the layout.master, you can use the "searchresults" CSS class added to the body on the SearchResults.aspx page to change the image.
Something like this if the element the background is applied to has a class of "pagewrapper":
.searchresults .pagewrapper {
background-image: no-cols.jpg;
}
If you are using an artisteer skin and it has a bunch of :after and :before pseudo-elements, your CSS is going to be (unnecessarily) more complicated.
Posting a link to your site will help us help you.
Thanks,
Joe D.