Thread:Raylan13/@comment-499247-20181125203534/@comment-3183460-20181126163150

I'd go to our development team requests page and ask for help there.

Essentially you'd take that image and overlay it on another image of a solid color, so that your image would appear on both sides, with a gap in the middle. The gap is where the page's content would sit atop of. Then you'd merge it all together, add some blending, then add it to the site via My Tools -> Theme Designer. Just make sure "fix" is unchecked so it scrolls. Adding it through Theme Designer shouldn't (I think) interfere with page-specific backgrounds.

The reason I link to the team above is because they do this sort of thing all the time and can do it much faster than I can ;)