How does Sontag present the argument that pictures take on different meanings and interpretations based on who is doing the viewing? Give specific examples.
If by "pictures" you mean photographs, Sontag has a complex theory of of photographic representation that she describes in her book On Photography. For Sontag, a photograph is best understood as a kind of physical artifact, distinct from a painting in that it the the result of recording light waves reflecting from the subject; she compares it to a death-mask, that is, a representation derived from the thing itself. Photographs in her view transcend or commodify the reality they represent. The value of the image, quite independent of the thing itself, is derived from its being captured on film. Sontag argues that this is very different from Plato's attitude toward representation:
It suited Plato’s derogatory attitude toward images to liken them to shadows—transitory, minimally informative, immaterial, impotent co-presences of the real things which cast them. But the force of photographic images comes from their being material realities in their own right, richly informative deposits left in the wake of whatever emitted them, potent means for turning the tables on reality—for turning it into a shadow. (p. 141)
It is the "material reality" of photography that makes them so utilitarian, and open to interpretation depending on context. A photo of a couple having sex could be "evidence" to a private eye, or a source of pleasure for a voyeur. The use, or meaning, of the image, is independent of the actual artifact.
Sontag explores this idea in more detail in her book Regarding the Pain of Others. Here, she considers how photographs of war atrocities might affect different viewers. One person might react with shock and disgust; another might react with anger and a desire for vengeance; a third might grieve for the victims. In a development from her earlier view of photographs as material objects, here Sontag sees empathy as a fundamental resource for understanding these pictures, even though the sheer number of these images, and the emotional distance photography provides, can make an empathetic response difficult.
You can find a detailed eNote on On Photography here.