In short:
all image on Wikimedia Commons are Free. You can thus use them for any purpose provided that you 1) give proper credits to the author and 2) mention the licence; the GFDL also requires that you put a link to the text of the GFDL licence.
The licence of your web site will not by jeopardised by the use of a GFDL image: the licence applies only to the image, not to the page itself. The so-called "viral clause" of the GFDL and other Free licence means that derivative of the images will be under the same Free licence as the original; this does not apply for a web page, since it is not a derivative of the image.
Cheers ! -- Rama