I swear I have searched and tried everything I could find, but it isn't working for me for some reason.

I have added a new widget area in my header section.
<?php bfa_widget_area('name=swheaderfullbanner&cells=1') ; ?>

I have added my ad code into the widget and it shows up in the header. Now I just want to center the ad image that shows up. I tried adding CSS in "add html/css inserts", but I'm not sure if I have the correct CSS. I've tried what has been posted already.

Here are some of the variations of the CSS I have tried. Let me know if you know one of them should work. When I look at my source code I see that the "swheaderfullbanner" is a div. I'm still a little unclear on when to use '#' and when to use '.' in my CSS.

div#swheaderfullbanner{margin-right: auto;margin-left: auto;}
div.swheaderfullbanner{margin-right: auto;margin-left: auto;}

Did something change with 3.4.4 (or an earlier version) that would make this not work?

I originally had my theme doing a fluid width (99%, 600 min, 900 max), and have recently tried doing fixed width (900, 600 min) after reading another post and have no change in behavior.

Thanks,

Scott

What is the URL?

http://www.shootwireless.com/

Try adding this to ATO->Add HTML/CSS Inserts->CSS Inserts

#swheaderfullbanner  {
margin-left:auto;
margin-right:auto;
width:40%;
}

Thanks for the tip. The code you provided worked OK for the small ad image I was using. When I changed it to 50% from 40% for the width the small image was centered and stayed centered when I resized the browser window. I am back to using a Fluid layout width.

Then as a test I changed to a larger ad image and the image was no longer centered. Then I realized what was going on. I wanted the contents of the swheaderfullbanner DIV to be centered, not the DIV container itself.

So, I ended up using this for my added CSS:
#swheaderfullbanner{text-align:center;}

That centers my large and small banner ad images inside the DIV container, which stays at 100% of the width of the layout, even when resizing the browser window.

Do you see any issues using that text-align style to keep my image centered within the custom widget area?

Nope, I don't see any problem. Glad it's working for you. - turnd out that when you only have one cell in a new widget area, this is how you center it because it is a DIV. If you have more than one cell, you get a TABLE and then you can use the 'align' property of the new widget statement.