Being able to do rules like this would be great, and (i think) a clear solution to the atomic positioning you mentioned:
background-image: url(a.gif);
background-anchor: 0 150px;
background-position: 0 0;
background-image: url(a.gif);
background-anchor: center center;
background-position: bottom right;
background-image: url(a.gif);
background-anchor: 100% 100%;
background-position: center center;
background-image: url(a.gif);
background-anchor: 50% 50%;
background-position: 50% 50%;
If an background-anchor wasn’t specified, or was set to ‘auto’, then it would default to today’s functionality where it basically mirrors the background-position.