The easiest way to add a background image to your Help Center is in Admin > Help Center > Help Center Design and click Edit Custom CSS to change the CSS of the Help Center.
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Add the following to the template:
.dp-po .dp-po-main {
background-image: url(your_url_here);
}
You can add you can add background-size: contain;
or background-size: cover;
to adapt the size as needed
The URL needs to be a publicly accessible image. This will achieve a result like the below: