Sometimes, paintings will be displayed offset by about 1 block and therefore seem to float in mid-air, though their hitboxes are still in place (attachments 1, 2, 4, 6 & 9). Other times, the whole entity (along with it's hitbox) will move 1 block up (attachments 3 & 7).
The original painting positions can be seen in attachments 5 and 8.
Works facing all directions.

NOTE: The world I originally saw this in was uncommonly redstone-heavy and I was unable to recreate the bug in another world.

Tested in Minecraft versions 1.8.3, 1.8.4, 1.8.5 and 1.8.6.

Sorry if this is a duplicate, i looked through all issues with the word "painting" in their name and didn't find anything similar.