Hello,
This is set below in your template.css file at line 999. Just remove the background line of CSS:
#s5_tabshow_left {
background: url("../images/Shape5_MaxedMag_static.png") repeat scroll 0 0%, rgba(0, 0, 0, 0) -moz-linear-gradient(center top , #676767 0%, #000000 100%) repeat scroll 0 0;
box-shadow: 0 0 4px 2px rgba(0, 0, 0, 0.5) inset;
color: #c2c2c2;
margin-top: -11px;
overflow: hidden;
}