I hate it when you go searching forever on a problem....then you ask a question...and 2 minutes later you get an idea...
I didn't think this would work because background-size is a CSS3 attribute...but it does...at least in Sigil.
I have this as my cover.html file:
<body style='background:url("../Images/cover.jpg") no-repeat fixed center center; background-size:contain'>
Of course kindle Previewer doesn't support Max-Height and Max-Width...it strips them out...and ADE displayed fine when it opens...but then barfs when I use the TOC to go back to the cover...Sony Reader accepts it, but doesn't center the image...
so...there is hopefully a better way of doing it??