Is it possible to have different logo images for different categories via css. My site is www.intervenors.ca

NO and YES,
no not directly
yes you could do it by removing the %logo adding in your own code and removing the <img...> statement that displays the logo and adding a 'style="background..." like you see for the header image container. you may have to play with th class names, but if you experiment a bit, you should be able to do it.