A Post with an Image
The header section above has small and large CMS images set as the background, This will be used as a fall back for posts with no image.
Original Blog Post Image is on the right.
Sections Pro is set to hide this image when the page is viewed and use its location as the background in the other sections instead.
You can optionally display this image as well if required - perhaps in the header as a small image, with a blurred version as the background.
The following Section has a normal (NON CMS background image set in its background child stack. For posts with no image you should see some blue swirls.
A Post with an Image
The following Section has a background child set to a gradient as the fall back. For posts with no image you should see a gradient below.