technically you could make the image the Pasteboard image with no Page background and set it up to fill the browser.

I find this does not work.
Rather frustrating Acorn, it works in v19 but not in the current Pro+ version; somewhat a backwards step and should be flagged to Xara. I discovered this a few days ago; I had to go back to v19 to set the pasteboard and then reopen it in Pro +, which does work, even with 5K screens