Two conceptual distinctions should be borne in mind. Firstly, some representations refer to particular things, and some refer to no particular thing. For example, a portrait has to relate to some actual person, but another picture can be a picture of a woman reading a book, without there being any particular woman or book which it is about. Secondly, there is a difference between a picture's standing for something as a symbol and its depicting something. In a painting, a lamb may stand for or symbolize Christ, but what is depicted in the painting is a lamb.
Depiction is an utterly familiar practice which proves hard to analyse.
The apparently common-sense idea that the surface of a picture resembles what it depicts is usually rejected by philosophers. They try instead to specify the state of mind of someone who both sees the surface of a picture and understands what it depicts. Different accounts invoke the notions of 'seeing a woman in' the painted surface, or making believe that our seeing the picture is our seeing a woman, or experiencing a resemblance between the picture's appearance and the two-dimensional appearance which a woman would have if we saw her literally. This is a challenging area of philosophy, which has to negotiate the sophistications of contemporary philosophy of mind, while remaining alive to the history of representation in the arts and the interest which pictures actually have for their audience. R. Wollheim, Art and its Objects , 2nd edn.