Floating Glitch
In this case they up a partition, then put the portrait on the back of it, then removed the partition, leaving the portrait facing backwards.
Edit: I just noticed that guide is specifically for floating table top items. Floating wall hanging items is the same basic concept. You put the portrait on the partition, Turn the Toggle Counter Surface Placement button OFF, click on the portrait and then right-click (or hit escape) to cancel, then move the partition out of the way, and the portrait should be hanging in mid-air.