Did you copy/paste the code from above, or use Sigil’s "add cover" tool?
If you copy/pasted the image or viewport dimensions may be incorrect.
The add cover tool will automatically add the correct image size and viewport dimensions so you will get the image to the largest size (maximizes height OR width depending on orientation) possible without changing the image aspect. You will have borders along either the side OR top, but not both.
I manually add a style to change the background color to an appropriate color to compliment the image like this:
Code:
<body style="background-color:black">
OR
<body style="background-color:rgb(0,0,0)">
OR
<body style="background-color:rgba(0,0,0,1)">
You can use an image editor eye dropper tool to get an appropriate color directly from the cover image if black or white don’t work.