Replace background with “Banner Image”
Optoin available in RapidWeaver 8

Override Site Banner
In RW you can replace the current background. Just drag your image in the field.

Edit background with Image editor

Open the Theme content and all images ( contents/images/editable_images/image_…jpg).
Add your image with “resources” option in RW

You can replace the image ( add another image) with the “resources” option of RW.
Page inspector/header/css/….
#global_width, #sidebarBackground { background-image: url(../../resources/image.jpg) !important;