Not sure which version of Zencart you are running??
BUT
Visited site..view source in browser reveals
includes/templates/template_default/images/logo.gif
(not using classic folder)
<link rel="stylesheet" type="text/css" href="includes/templates/classic/css/stylesheet_new.css" />
<link rel="stylesheet" type="text/css" href="includes/templates/classic/css/stylesheet_original.css" />
You are running 2 stylesheets, so one maybe overriding another.
How you proceed depends on stylesheet/s codes/version of
zencart being used.
HTH