
Alfred Stieglitz - "Fifth Avenue 1892"
Postmodernism evolved because artists/photographers/musicians became frustrated with the boundaries and rules of their field. They pushed theses 'boundaries' and broke the 'rules', and now there are no restrictions on what an artist can or can't do. Postmodernism is about combining old methods, with modern ways of thinking, and allowing the photographer/artist/musician to decide the limitations, if any at all.
An example of a great modernist photographer is Alfred Stieglitz. I only ever learnt about him and his work because I've been a massive fan of the painter Georgia O'Keefe for years and years, and Stieglitz is most well-known for his discovery of her, and their marriage.
Stieglitz amazes me so much because his work was so different to the other stiff and retouched images (taken by other photographers) that surrounded him at that time. His work included movement, and he would often stand in very bad weather just to get the shots that he was particuarly looking for.

Jackson Pollock is a good example of a postmodernist artist.
"When I am in my painting, I'm not aware of what I'm doing. It is only after a sort of 'get acquainted' period that I see what I have been about. I have no fear of making changes, destroying the image, etc., because the painting has a life of its own. I try to let it come through. It is only when I lose contact with the painting that the result is a mess. Otherwise there is pure harmony, an easy give and take, and the painting comes out well."
His work and his view on "art' is a perfect example of postmodernism. He completely rejected any form of boundary, rule, limit that had been put there before, and worked completely how he wanted to work, and produced work he wanted to produce, rather than sticking to some ancient rule book of art.
Modernist artists tend to take the approach of attempting to make their work mean something, to have some sort of affect on world, to change it? Where as postmodernists do whatever they please. not for a reason as such, just because they want to, to express themselves and for others to express themselves.