I am attempting to have an image that I have in my background pasteboard stay fixed to the screen and not scroll up and down with the content of my site. Unfortunately the documentation on this seems rather vague.
Normally if you have a long web page that can be scrolled in the browser, the browser background scrolls with it, so the page content remains in the same place relative to the background. But you can 'fix' the background so it doesn't scroll with the page content. Right-click on the pasteboard and select "Pasteboard background" > "Fixed". Then when you preview, the content slides over the fixed background as you scroll (to see this, make sure your page is long enough so the browser shows a scroll bar!).
Where is this magical "Pasteboard background"? When I right click on the background I don't see anything that would allow me to pick between Fixed and Scroll. But then after about a half an hour of messing around I discovered that you HAVE to "Lock" that Pasteboard background layer in the "Page and Layer Gallery" list or that item won't show up when you right click the background of your composition. PHEW! Once that layer is locked you can right click on the background and pick between Fixed and Scrolling.