You have to attach the images as a Resource folder, then attach a .css file with rules for each page, e.g.,
.page[data-name="1"]{
background:url("../resources/page-bg-1.png");
}
.page[data-name="2"]{
background:url("../resources/page-bg-2.png");
}
Etc.
If that's outside your comfort zone, I would suggest connecting with a web developer. I'd be happy to recommend one. It shouldn't take them more than an hour or so.