I keep trying to cure people of bad habits. Silly me.
Full-page images used as background usually look awful. They distract from the album itself. If they're not large enough, they get blurry. If they are large enough, they slow down page loading something fierce.
But, if you're bound and determined....
background-position: center center;
Add the image on the Custom panel, as if it were a simple texture file.
And you are spot on about full page images, bad habits, etc. etc. I'm experimenting with Pluto with the idea of switching some things over, including full page images - but I haven't arrived at what I'm looking for. This is likely compounded by I'm not sure what I'm looking for other than simplicity, good images, quick loading. I'm getting closer but until I arrive at the new destination, using full page images will remain as a "temporary" fix. Thanks.