OK, if you really want to use CSS3 multiple background images, you can add those images to the container div (assuming the container has a class of "container"—so change to suit):
background: #000000 url("top-right.png") no-repeat right top, url("bot-left.png") no-repeat left bottom;
background-repeat: no-repeat, no-repeat;
background-position: right top, left bottom;
If you want it to work in older browsers, you could instead wrap the container div in one or two divs and put each image separately on them (one div if you don't already have a background image on the container, two divs if you do).
Of course, change the background color to whatever color the container actually has, if any.