2.17 What a picture must have in common with reality in order to be able to picture it in its way -- rightly or falsely -- is its form of representation.

See 2.151. This form is a possibility. For x to be a picture of y, x and y must share the possibility of being true or real (including, I suppose, being truly non-existent). One must be a logical possibility, the other must represent that logical possibility. It is that possibility that they must have in common. But what kind of thing is that? Are we reifying possibility? Making logic metaphysical? Surely not. So what is being said? Could it be: nothing?

Ostrow (p. 44): “Far from being imagined as a third element, stateable or unstateable, the pictorial form is no element at all, but rather part of the picture’s way of depicting. The picture “must” have in common with reality its particular pictorial form precisely because this form is constituted by this picture’s application to the world – just as the possibilities of length are given through the ruler’s use in measuring magnitudes.”

See also 2.033.