There's no option for this within Q2A. If you want only the login to be https, you could set this up at the server level or htaccess (i.e. redirect the login/register pages to https, redirect everything else on https back to http).
But the best option is to move everything to https. If it's insecure to send password details over http then it's also insecure to send your cookies over http too, once you are already logged in.
Images can be a problem but currently all browsers will load http images on a https page. If all images are 'local' (i.e. on your site) then you can rewrite them all to https://
It's more difficult for external images because you don't know whether the external server supports https. On my site I set up a "proxy" PHP script that fetches the image over http, but the URL is a https one on my site. I may release this as a plugin in the future if people are interested.
