scrollbars - lots of people do it, but yeah, not supposed to alter anything outside of description area.
With some work, the template could be made to be auctiva-compatible. I'm thinking along the lines like wahm said, of create a table that is 100% wide and 100% tall and contains rest of template and use the background image as background in this larger table
basically, instead of body tags
<body background="image.jpg">
template info
</body>
you'd have
<table width=100% background="image.jpg">
<tr>
<td>
template info here
</td>
</tr>
</table>