I'm afraid that I don't feel like it's my responsibility to ask people to switch to https. There are all kinds of image hosts and servers which have not switched over yet. For example, in my other forum, someone uses TinyPic to host their images, and none of their images use https URLs. So I don't have time to contact everyone who doesn't use a secure image host. I have my hands full with my server/sites and Inkscape projects. The way I see it is this just an uncomfortable time on the internet, while we wait for everyone to catch up. Eventually SSL will be ubiquitous (?? wow, where did that word come from, haha! Sometimes I'm banging my head to try and think of the word which I can't remember, and other times, strange words just pop out!).
I've never heard of it being free. As far as I understand, you have to buy an SSL certificate. I guess I've seen some hosts which offer it for free, but to my knowledge, that's only because that's how they have configured their services. They still have to pay for it.
It looks like that site/service (which you referenced) is depending on donations to pay for the certificates. Good for them! But will they always be able to do this? We could have a downturn in the economy, and they go out of business. Then what?
Just to be clear, I'm not talking about disabling our SSL. I've just disabled the malfunctioning forum feature (image proxy).