Or maybe not… I got black backgrounds/images for a few minutes, but now theu seem to show up. So maybe it was just a local cache-problem that didn’t pick up the change immediately.
It turns out that the Photon-setting (“Speed up images and photos”) of Jetpack moves the images to i1.wp.com. I turned off Photon, and now the images are server locally from tjomlid.com. But still the same problem persists…