It seems you are assuming a white background- the background should match the color the add is placed at.
For my preview, the add was placed partially in actual empty space and partially on empty space that had a background image.
We have two detection modes (http://www.emptyspaceads.com/publisher/customization_options) that you can use. The preview feature is using the Automatic detection which isn't always perfect. However, you can use the "Let Me Decide" detection which will only cause the ads to appear in divs or HTML elements that you add the class attribute "ESAemptyspace" to. That way you can explicitly tell us where you'd like the ads to appear on your site.
