To replace the "header" image, go to the theme index in your admin section and find:
echo '
</td> <td class="windowbg" width="100%">
<img src="http://www.whipevents.com/images/viperhome.jpg">
</td></tr></table>
Then just replace the image with yours. Sorry, I know that isn't as smooth as it should be. You can insert it into your images folder in the template and replace the whole thing with "images/image.jpg"