In a few cases i'm updating things automatically, the example above, i replaced already all topics with i.imgurl.com to the https variant, but it won't work for all image hosts....
A popular filehost tinypic.com does not support https, so it won't show any images.
You could try to upload your pictures to another host that supports HTTPS.
SMF supplies a "proxy" for images, that auto downloads any unsecured image and servers it from our webserver, but for some reason this isen't working.. i'm looking into that.
The 6te topic in this example had i.imgurl.com images and also tinypic, the imgurl pics work now.