When you begin to examine terms like "originality" with reference to those constructive situations to which they do in fact analytically apply, the nature of the description that they provide tends to reduce the imitation-invention ratio in a work of art quite properly to the simple matter of a statistic. Within this statistic, no work of art is ever genuinely "original" -- if it were, it would be unrecognizable.
The point is that originality is opposed to comprehensibility. Something we have never seen before can hardly convey meaning. A Rorschach blob carries "meaning" only insofar as we think it resembles things we have seen before. A new visual idea is best placed in a context of familiar ideas and quotations which can do the work of carrying meaning. The second time we see the idea, perhaps it can carry meaning or otherwise evoke, because of the context in which we first saw it.
Photography, as a visual art, can draw on a world of sources. While photography certainly does draw on its own history as a source of idioms and ideas, as a source of the comprehensible, it can also draw on other visual arts as well as mass culture. In a very real sense, many of the arts build new work on a foundation of the culture as a whole, as a source of ideas and quotations that the audience already understands.
Photography as art draws on other forms of photography, popular/mass snapshot aesthetics, commercial photography, as well as other forms of art. The more I think about it, the more I think that the current fashion of HDR and popped color draws on comics, graffiti, and video games. Arguably the muted colors and contrast-to-the-midtones fashion work we see these days is also a bit of comic book styling, albeit without the bright colors.