Just saw this on social media. Thought would be nice to share with you guys here
https://blog.sucuri.net/2016/04/sucuri-firewall-free-letsencrypt-ssl-certs-for-everyone.html
Note that you'd need some kind of image cache or proxy like http://addons.elkarte.net/enhancement/Image-Cache.html
I'd go for a proxy approach. See
here (https://github.com/Frenzie/Extensions/blob/e820c338b2a37a4ac01a578722644b32ba927aea/xExtension-ImageProxy/extension.php#L24-L36).
In the last couple of years a couple of projects to provide free certificates have emerged.
To me, https is not yet a priority, in the future I'll consider it.