Hi, I'm new to css and this forum thats why i'm posting my question here.

I've built and am maintaining a website http://www.motionsgarden.se/ndex.html

The site has a picture gallery located under "bilder" in the menu and that gallery has subsections
with each visiting group or school class having their own page with pictures.

Now to what i need help with.
I used this guide to add a dropshadow effect with the help of css:
http://www.alistapart.com/articles/cssdropshadows/

And i was successful in adding a shadow to one image.
Tho i would imagine to make the process as efficient as possible i would need to have
an external .css file with the code to add the shadows to every picture in the subcontent of the gallery.

How do i go about doing that?