You can set a custom background image on most pages
Simply copy the below code into the HTML view of a custom page and target the pages you want to display the custom background.
<style>
TARGET PAGE ID EG(#custompage_2Content, #aboutContent, #custompageContent) {
background-image:url("IMAGE URL");
background-position:fixed;
background-size:cover;
background-position:center center;
}
</style>