I think that now, you just need this part: .headerimage {
position: relative;
width: 500px; /* Width of the banner, in pixels */
height: 220px; /* Height of the banner, in pixels */
margin-left: auto; /* To center image leave as auto */
/* To right align leave as auto */
/* To left align change to 20px */
margin-right: auto; /* To center image leave as auto */
/* To right align change to 20px */
/* To left align leave as auto */
margin-bottom: 20px; /* Space between bottom of image and outer box of style */
background-image: url("http://pics.livejournal.com/cyrnelle/pic/0000xh6c"); /* Header image URL */
background-repeat: no-repeat;
}

Just change the width, height, left/right align, and margin-bottom to what you want, and replace the url by the url of your banner. And copy-paste this in the Custom CSS/Custom Stylesheet.

That should work. If it doesn't, tell me, and I'll do a .txt for you. ;)