If you are using the background image for the rounded corners then I would rather increase the padding style of the main div to give enough room for the rounded corners of the background image to be visible.
Try increasing the padding of the main div style:
#mainWrapperDivWithBGImage
{
background: url("myImageWithRoundedCorners.jpg") no-repeat scroll 0 0 transparent;
height: 248px;
margin: 0;
overflow: hidden;
padding: 10px 10px;
width: 996px;
}
P.S: I assume the rounded corners have a radius of 10px.