In On Photography, Sontag argues that photography intrinsically objectifies our world and, by doing so, reveals highly convincing artifacts of moments we've never actually witnessed—and that has expanded that world, extending it outward infinitely. It provides a viewer with an icon or representation of an event, even if this moment is one never actually witnessed first-hand. The act of taking, and looking at, pictures is one of editing reality, in terms of manipulation of space, time, and other mediations of natural perception. "Photographs...fiddle with the scale of the world," with images "doctored and tricked out." This, Sontag proposes, has created a "grammar, an ethics of seeing. A new visual code."

The effect of this can perhaps be roughly summarized as, to what extent do you (and can you) believe in what you see? Does processed imagery have more influence on thought than other media, such as painting, or even motion pictures? Photography, Sontag writes, seems to be direct visual evidence. "Now [photographs] provide most of the knowledge of the look of the past, and reach of the present." Objects, events, and novelties become anthologized as part of our mind-bank. Naturally, a subjective reading of a given image relies on its audience's values; a policeman sees an image as incriminating, and a tourist sees a postcard as a sentimental moment in time. And with a single image, or a series, the viewer is free to imaginatively expand on its subject or scenario and so—in a sense—occupy that world at will. "Nothing holds readers to the recommended order or indicates the amount of time to be spent on each photograph."

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 eNotes page on On Photography here.