I could probably find a setting in my browser (firefox) to make them show, but images hosted on another site that use http don't load on forum thread pages (presumably because the rest of the page is https). If I right click on the broken image holder and click view image, it shows the image ok. I was thinking the site code could rewrite all image urls hosted on external sites to include the s in https. Maybe images that are on non secure sites won't load, but I'm thinking that's a pretty small number. When I include an image in my post and upon posting I see the image only has a broken image holder, my first thought is to look if its http, if it is, I just edit the post and add the s to the http in the image url, and only a couple times ever has the image refused to load. Seems like sticking in the "s" would be fairly easy coding wise.

There's a browser extension called "HTTPS Everywhere" that supposedly forces https by default.

I don't know whether it will work with in-line images however.

YMMV.

https://chrome.google.com/webstore/detail/https-everywhere/gcbommkclmclpchllfjekcdonpmejbdp