Hmm it's possible the author of the skin just so happen to include the direct url to the image rather then then a phrase. Just look through your header(most of the time) template and see if you cant pin point if there is instead a more direct url like <img src="/images/logo/logo.jpg"">
Another thing I sometimes do is right click on the standard logo, and find the image path, then just upload my own images to the server over writing the default one, and this way no customization is needed usually.