Currently it is not possible. I will add it but optimization of png takes too much time. I will implement it when I find a better way.
How can I enable this? I get 500 error when its enabled.
*** Attention ***
If your server does not support mod_expires.c, this option can cause the Internel Server Error (500). If you see such error please do the steps below.
Open .htaccess and remove this rule
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault A0
ExpiresByType image/gif A2592000
ExpiresByType image/png A2592000
ExpiresByType image/jpg A2592000
ExpiresByType image/jpeg A2592000
ExpiresByType image/ico A2592000
ExpiresByType text/css A2592000
ExpiresByType text/javascript A2592000
</IfModule>
You need to contact to your hosting provider. In your system mod_expires.c library hasnt been installed.