One of my favorite themes is how many photographs there are out there. Arguably, all the photographs have been made. If you don't believe me, try uploading an image into google's image search. As of this writing, it will pull out a bunch of what it terms visually similar images, and that's just exactly what it does. Given that it always seems to scrape up some startling stuff, and given that google probably has some infinitesimal fraction of All The Photographs indexed, we can learn something from this.
If every photograph has been taken (and yes, I know there are several dimensions in which this isn't and never well be true, bear with me) we still have the fact that not every photograph has been placed next to every other photograph. If we assume that there are, really, one million genuinely distinct images which can be made, then there are 1,000,000,000,000 possible pairings of images. Not all of these pairings make sense, but there seems to be a lot of potential here.
A pair of images, presented together as a piece of art or as decor, potentially says something different or at any rate larger than either piece alone. Collages, portfolios, shows, these are all new constructs which can be fundamentally new and interesting even if they are made of up things we've seen before. This suggests that one could create a show or portfolio out of other people's photographs, and yet still be making art. Copyright issues aside, of course. This has, of course, been done. Curators and editors have long been with us, perhaps we should promote them to artists? I have argued that Gary Winogrand and Vivian Maier might as well be constructs of their curators.
The general notion of a body of work, of multiple images (or pieces of art) juxtaposed is a theme I'm thinking over a fair bit these days. It's more important and central than I thought it was.