Cache buttons etc images

[Andreas]

That doesn't make sense at all, as the images never change (except if you upload new ones).

But if you really want to do that put this as .htaccess in the appropriate directory

ExpiresActive On
ExpiresDefault A0
Header unset Cache-Control
Header append Cache-Control "no-cache, must-revalidate"

Please note that this requires Apache with mod_expires and mod_headers