Right, thanks. So now I'm slightly more with it than the help pages, the drill is
To set a background on a space page
1. ignore help page
2. go to image page
3. attach image to space page, if it isn't already. (If space pagename contains a comma, fail here.) Image appears in sidebar.
4. Still on image page, set image as background on space page.
5. detach image from space page. It disappears from sidebar but remains as background.
1. Go to image page, if you can find it (the hard part)
2. Attach image to space page. This makes the space page appear in the checklist above the "Set as background" button
3. Still on image page, unset as background (untick)
4. Detach image from space page
Or, as Melanie says
1. Go to any image page
2. Attach to space page
3. Set as background (replacing previous background)
4. Unset as background
If the space page is on your watchlist, you can use the watchlist images page to get to the image page.
Otherwise, there might be no way to find the image page, except a browser feature. MS Edge doesn't have a "Save background" command, so you have to go through the "Open in Internet Explorer" routine.