If the round-cornered gif has a transparent background and is the same color as #contentMainWrapper, that would explain your dilemma. You want to shift it so it is higher on the screen than its container. Try
position: relative;
top: -10px;
in this rule
Code:
/* ROUND CORNERS By SFS JAY */
#mainWrapperTop {
background:transparent url(../images/main_wrapper_top.gif) no-repeat scroll center bottom;
height:10px;
width:950px;
margin:0 auto;
padding:0;
text-align:left;
position: relative;
top: -10px;
}