You can only use the templates already provided with the Single Page Site. You will be able to add any text you want and up to four images into the page area of the template but you will not be able to edit or replace the templates. If you have a pre-designed page that you want to use simply select the plain white template, upload the image file and then add the image to your site.
The template you select will only be viewable when you browse to your live website. It will not show when using the preview option of the single page site.