Expires module in Apache solves this
a2enmod expires
it needs to be loaded in server config, and set up in .htaccess (or in server config).
With an Expires header, the resource is only requested the first time. Before the expiration date, subsequent requests are fulfilled from browser cache. After the specified time expires and the resource is needed, only then it is requested again (conditionally – a 304 will be returned for an unchanged resource). The only reliable way to clear it from the cache before it expires is manually, or by forcing a refresh (usually Ctrl-F5). (This could be an issue if the resource changes in the meantime, but statical images don’t change very often.)
# enable the directives - assuming they're not enabled globally
ExpiresActive on
# send an Expires: header for each of these mimetypes (as defined by server)
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
# css may change a bit sometimes, so define shorter expiration
ExpiresByType text/css "access plus 1 days"
For favicon.ico, a bit more work is needed (Apache normally does not recognize Windows icon files, and sends this as the default text/plain).
# special MIME type for icons - see http://www.iana.org/assignments/media-types/image/vnd.microsoft.icon
AddType image/vnd.microsoft.icon .ico
# now we have icon MIME type, we can use it
# my favicon doesn't change much
ExpiresByType image/vnd.microsoft.icon "access plus 3 months"
And voila, It Works™!