Hi jwitters
You can override your own image editing this file /templates/gk_musicstate/css/template.css around line 196
.frontpage.dark-bottom #gkDarkBottom {
background: #1e1e1e url('../images/common/dark_bottom_bg.jpg') no-repeat center center;
bottom: -450px;
height: 1000px;
position: absolute;
width: 100%;
z-index: 0;
}