Images use http

You mean that the links use "http" instead of "https". This is quite different. You can view that image with HTTPS just fine (my browser does that for me, since I have HTTPS Everywhere installed). It's just that "http" is being used in the link, so most people are getting it in the insecure way.

Funny, I also have HTTPS Everywhere but the image URL is https for me.

It's the thumbnail that's giving the warning. If I click on the image it's https when large. Funny.

I'm doing some changes to the current trisquel web infraestructure, so those changes could be caused by me.

My intention is disable all http content, and redirect all http requests to https, but seems like not all requests are following my rules, I will try to debug the issue later today.