It came to my attention recently that images folks were adding to posts which are hosted on non-secure sites were breaking here.
I've done a bit of research around this and it looks like our security certificate expired, which I've renewed this morning. It should take affect in the next few minutes.
My hope and expectation is that the certificate will fix this issue for you all going forward.
One way to avoid the issue entirely is to use the forum's image uploader tucked away in the "Attachments" link below a post's edit form. But if you want to continue to use the IMG tag that shouldn't be an issue. We should know one way or the other here in a bit. Sorry for the jankiness.