If you post an image of your original piece of furniture on Instagram either as a static image or video, and you watermark it or add your copyright symbol, is the design of the object in the image protected, or is it just the image of that piece that is actually protected by doing that?

Your work is protected as soon as you create it so your video or picture is evidence of your unregistered design rights – this is what you should upload to the ACID IP Databank. The images themselves will then have their own copyright but they don’t affect the design rights in the actual product.