The "logo.gif" IS controlled by several stylesheets. Four for just the logoWrapper itself. Of course, not all control the width, height, centering, float, etc.
With so many stylesheets controlling an area/item, it's easy to make a change for something and it also affects something you didn't want to happen.
I can make the image center with the stylesheet BUT, in our case, I had to change three settings.
In your case, it may be as simple as adding a call to the bottom of the stylesheet.css like
PHP Code:
#logo {text-align: center;} 
We could do much better than a WAG if we had a link to your site.
If you use Firefox or (my current fav) Chrome for a browser, get the developer extensions installed. They really help you "see under the hood."