It looks like it might be inheriting css you cahnged in a parent container maybe?
Try making your banner anchor look like this:
If you create your banners in the built in Banner section of Joomla it will place you banner in a DIV
class="banneritem-none" then modify the link for your banner as above to match your template by entering
class=boxed to the anchor tag. If you must specify dimentions, only enter the width and without the quotes as above...