If we take into the consideration, that Waldo’s posture is not known, then by cutting it out of paper and sharing it with the other party we reveal this information

This is true. However, some versions of Where’s Waldo books already provide you with Waldo’s posture in that scene (in a side panel), so you are not giving away additional knowledge.

Otherwise, if the posture of Waldo is previously known to other parties, then even without knowing his location one can prepare such a cutout with high probability, without knowing the real Waldo location.

The cardboard cutout alone is not the proof. It is the cut out + it overlaying on top of the original scene that comprises the entire proof.