Yes, optional is fine.

Reproducing images at the "right" size is a very niche requirement. Most images don't even have a right size, including all photographs unless they either use an ortographic projection (they won't) or the object is completely flat and parallel to the focus plane (with recilinear projection, which most normal camera lenses come close to). Having this as a standard attribute in a generic image format makes no sense.

You are complaining about a format - with the stated goal to be very simple - not supporting some scientific usecase. What...? Use a different format then. That's not what this is trying to be. In fact, it's the exact opposite of what it's trying to be.