You need to put the picture into your ebook. In Sigil that would mean clicking on the "Add Existing" (plus sign on toolbar or File/Add in the menu) and selecting your file.
Once the image is in your epub, then you need to reference the image using this format:
background-image: url("
../Images/example.jpg");
where
../Images/example.jpg is the full internal path to the image file you added.
You can (and should) spend some time looking through some of the CSS/HTML tutorials found on
our Wiki or at
W3Schools