[Lynne]

If you go to Styles & Templates -> Style Manager -> My Style -> Main Css, at the bottom is Additional CSS Options. Add it in there. Don't bother creating a directory named cylic, just use one that already exists. You probably have image/misc already, so just use that. Then it would be
http://www.mydomain.com/images/misc/logostrip_bg.gif

In your header, you probably have something like this (this is default 3.8), you should add the part in red:

<div id="logostrip">
<table border="0" width="$stylevar[outertablewidth]" cellpadding="0" cellspacing="0" align="center">
<tr>
    <td align="$stylevar[left]"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></td>
    <td align="$stylevar[right]" id="header_right_cell">
        <if condition="$ad_location['ad_header_logo']">$ad_location[ad_header_logo]<else />&nbsp;</if>
    </td>
</tr>
</table>
</div>
<!-- /logo -->

I don't really know what the result will be here since I haven't tried it. I think I usually see people just add it into the table tag or the row tag. You can try that if this gives weird results - just add id="logostrip" into either the table of row tag - ie. <tr id="logostrip">.

Also, check the height of your background that you are adding and change the css height to be correct - ie. height: xxxpx;