To reproduce, go to your blogs.gnome.org site, click New -> Post from the title bar, then click the Add Media button. Note that all uploaded files are given an http:// URL (which can't be changed). This means it's impossible to use the images in a blog post without trigger a browser security warning, unless when writing a post I switch from the default Visual editor that I actually use to the Text editor and then manually edit the resultant HTML. (Just changing the links to https:// fixes the problem. I edited each image I've used in each post on my blog in order to get rid of Epiphany's security warning icon. Safari is the only noteworthy browser that won't show this warning.)

I guess this will be difficult to fix for articles that are already published, but it can be fixed for new articles/media going forward.