2.13 In a picture, the elements of the picture correspond to objects.

This is a strange sentence: To the objects correspond in the picture the elements of the picture. Why not just: The elements of a picture correspond to objects? For two reasons, I think. The first is to emphasize the correspondence to objects. The second is that the correspondence takes place within the picture. The picture, that is to say, is not just a copy of reality, a double. A shadow corresponds to whatever casts the shadow, but the correspondence between this object and its shadow is, so to speak, external. A shadow is not a work of art, like a portrait, intended or presented as a copy. Pictures, though, are like portraits in this respect. The correspondence is internal in the sense that the picture 'claims' to represent certain things. Also, bear in mind that objects are possibility spaces. The elements of a picture do indeed correspond to these. A pencil sketch can represent only in shades of grey, while a sculpture can represent not really color but three-dimensions of space (so can a sketch, of course, but in a different way). How a representation represents depends on the kind of representation it is. In an oil painting red will usually be represented by red, whereas in a charcoal sketch red will have to represented in some other way. The possibilities for the picture correspond to possibilities for reality (objects). Perhaps in fact this is all that Wittgenstein means here.