So when you first visit the site (or do a hard refresh ctrl+f5 to clear cache and reload) the site is not properly loading all CSS. That's apparent by the before/after screen shots below
After you load it the first time, it works perfectly, but every article loaded, for the first time, has its header image all messed up like this.
I greatly prefer not to give a URL
suffice to say im running 3.1.5, the gavick boutique module, unmolested, so it should be pretty easy to replicate.
BEFORE
AFTER