Ah sorry, I should have mentioned specifcity issue, along with allowing the <li> to increase in width to not cut off the image . Add this CSS in. Change classes as you see fit.
The first part widens the <li> to allow more width in. The second makes the image show instead of a white space that would otherwise be there.
.horizontalaccordion > ul li.active
.horizontalaccordion > ul li div.active /*body in there to give it slightly higher specificity*/
Now update your HTML
I used the same class="active" twice, though I differentiated it in the CSS using an element before the .active. I'd recommend changing the c lass to something different (whatever you want) . That should get it working.
<div class="active"><img src="/beta/images/toppbanner.jpg"/></div>