2.171 A picture can picture every reality whose form it has.
A spatial picture [can represent] everything spatial, a color one everything colored, etc.
A spatial picture, I take it, need not itself be spatial. That is to say, we can represent three-dimensional space without building a three-dimensional model, just as we do not need to use colors to represent colors (we can use words, for instance, instead). But again I am talking here as if pictures were to be understood fairly literally. If they are propositions, then what is a spatial picture? A proposition to do with space? Perhaps a proposition belonging to the set of all (possible) propositions about space? And then 2.171 becomes obviously true.