I don't think you can affect the banner using the CSS.  If you can, I have no clue how.  I'm sure there is an easier way, but the only way I could get it to work was to use the Banner Area section under Styles & Colors.  I ended up making a table and left justifying the image.  Replace 1000 with however wide you want it to be and put your image url in.

<TABLE>
<TR>
<TD WIDTH=1000px ALIGN=LEFT>
<img src="img URL here">
</TD>
</TR>
</TABLE>

I tried just using <LEFT></LEFT> tags, but it wouldn't work for me since the rest of my page is centered.  It seems the banner will automatically align with whatever is chosen under Styles & Colors > Page Styles.  You could left align the entire page using that.

FYI, my page doesn't actually reflect a left justified banner.  I did use it to try it out though.  I'm sure my guildies were wondering why a picture of my dog kept showing up randomly.

I figured it out.

.BannerArea {
    Custom CSS here
}