Do you know any way of replacing that banner with a different image?
Sure. First, create the image you'd like to use instead, upload it to your media library, and copy the image link.
Here is the original border image from the Luscious theme for reference:
http://s1.wp.com/wp-content/themes/premium/luscious/images/border.png
To figure out what CSS to use, I looked for any references to border.png in the original theme stylesheet:
https://s1.wp.com/wp-content/themes/premium/luscious/style.css?m=1344957909g&minify=false
I actually found two more selectors that I could have included before.
To replace the image, replace the url() values with the new border image you uploaded to your media library and add the it to your Appearance → Custom Design → CSS editor:
#wrap,
#subnav {
background: url("http://s1.wp.com/wp-content/themes/premium/luscious/images/border.png") repeat-x;
}
#comments h3,
.widget-area h4 {
background: url("http://s1.wp.com/wp-content/themes/premium/luscious/images/border.png") bottom repeat-x;
}
#footer {
background: url("http://s1.wp.com/wp-content/themes/premium/luscious/images/border.png") top repeat-x;
}