geee.
well the problem is the site_logo is a function in /include/functions.php search site_logo_img
as you can see the image its set to is $user->img('site_logo')) I dont see how you would set it to .jpg
so you could either wait for php genius to respond or...
1) open your overall_header.html ( in the templates folder)
2) find site_logo_img
3) change
<tr>
<td id="logorow" align="center"><div id="logo-left"><div id="logo-right">
<a href="{U_INDEX}">{SITE_LOGO_IMG}</a>
</div></div></td>
</tr>
to
<tr>
<td id="logorow" align="center"><div id="logo-left"><div id="logo-right">
<div id="logo-site"><a href="{U_INDEX}"></a></div>
</div></div></td>
</tr>
4) open theme/stylesheet.css
5) find logo
6)
add
#logo-site {
background: url(images/logo_site.jpg) top centre no-repeat;
}