Try using this place for backgrounds,
http://www.ifskinzone.net/applications/aio...kingen_simp.php
And this is how the upload the CSS sheet they give you,
Step 1:
Open up your Admin CP and visit Skinning & Styles > Manage Style Sheets.
Step 2:
Delete the CSS coding currently there. (That's your style sheet btw).
Step 3:
Copy (Ctrl + C) the CSS Style Sheet code of the skin you like. And then Paste (Ctrl + V) it into your Manage Style Sheets box.
Step 4:
Click the Edit CSS button and voila. New skin added. Unfortunately you cannot add more than one style sheet
As for the banner thing, not sure.