Some questions about caching

I am using apache webserver for a django application. How, do I set caching for images and css? I ran a webpagetest on my website, and it says "expiration not specified" for css and images. But, when I use Firebug, for css files, firebug show开发者_如何学Gos requests and response headers, as well as in cache tab it shows

Device: Disk

and no requests are shown for images. So, I am a bit confused. Whats happening here.?

I usually have this config for django static files in my apache vhost :

    Alias /static/ "/home/django/projectName/static/"
    <Directory "/home/django/projectName/static">
            Order allow,deny
            Allow from all
            Options +FollowSymLinks
            ExpiresActive On
            ExpiresByType image/gif A1209600
            ExpiresByType image/jpeg A1209600
            ExpiresByType image/png A1209600
            ExpiresByType text/css A1209600
            ExpiresByType text/javascript A1209600
            ExpiresByType application/x-javascript A1209600
            <FilesMatch "\.(css|js|gz|png|gif|jpe?g|flv|swf|ico|pdf|txt|html|htm)$">
                       ContentDigest On
                       FileETag MTime Size
            </FilesMatch>
    </Directory>

Remember to not use the django static.serve view in production.

Also use Ctrl+Shift+R to do a full reload in Firefox (override cache)