Summary: This will display an image (halloween theme in this case) above the sidebar.
Live preview: You can view it in action whilst it's up if you glance to your right by the sidebar http://www.bbsmiley.com/
Install: upload the image in your styles /overlay folder and then paste the following into your EXTRA.CSS template.
Code:
.sidebar .section:before {
content: "";
display: block;
height: 30px;
margin-top: 5px;
background: url("@imagePath/xenforo/overlay/pumpkin_sidebar.png") no-repeat scroll left transparent;
}
Additional Note: If you don't like the image being displayed ontop of the visitorpanel (personally I don't) then add this also to your EXTRA.CSS template.
Code:
.sidebar .visitorPanel:before {display: none !important;}