This is a fairly easy thought experiment to do, although the accuracy of the answer is perhaps hard to find. Take every photograph ever, from the View from the Window at Le Gras in 1826 to a picture of someone's lunch being uploaded to Instagram this very millisecond, and then select the picture in the chronological middle.
Several questions could be asked about this: does each frame of a movie count as a separate picture? Are photographs that are taken accidentally but never developed included? The technicalities are not important, since they probably wouldn't alter the answer outside of our great range of already existing uncertainty.
Photography was invented in the early 19th century, but cameras didn't become a consumer item until the early 20th century. Point and Shoot cameras became more and more prevalent, especially as the consumer economy expanded after World War II. In the early 1990s, disposable cameras were introduced, and in the early 21st century, digital cameras became consumer items. And it is the last point that is most important, because the availability of digital cameras has made the taking and storing of photographs exponentially easier. The median photograph was probably taken sometime after the popularization of digital cameras and smart phones. The actual year might be 2002, or it might be 2008, but it is probably quite recent.
One of the interesting things about this for me is that much of the 1990s, and into the 2000s, is undocumented in photographs for me. There is a website called http://vintageportland.wordpress.com that contains pictures of Portland, Oregon from its founding until the 1990s. Looking through the pictures, I realized that there are probably not that many more pictures of Portland in the 1990s then there are of Portland in the 1960s. The culture in Portland now, the culture of "Portlandia", was a nascent thing then, with vegan cafes and record shops just starting to spring up. But though this was the beginning of the current times, it falls on that other side of the great documentation divide. I won't be able to go back and find easy evidence of how we dressed, where we shopped, the way we looked, and the very quality of light itself are lost.
Part of the definition of the past is its inaccessibility. There used to be a joke about bathing, before indoor plumbing became a common technology: that people would bathe once a year, whether they needed it or not. At some point in the not too distant future, we might have trouble explaining that we used to only be photographed once a year, whether we needed it or not. When I was a child, the only pictures taken of me were school photographs and pictures at holiday gatherings. The past five years are the first time in history in which it is possible to have daily photographs of people doing mundane tasks. This is the first time that we will have photographic evidence of how people "normally" dress, how rooms are "normally" decorated. Our daily culture is finally revealed. Which will certainly be a boon to academic researchers in the future. But it will probably be an advantage to everyone, although perhaps a mixed blessing: if society manages to preserve itself, a hundred years from now people will be able to know about the daily lives of their great grandparents. But if the past is defined by its inaccessibility, what will happen when our current time will be recalled, at the touch of a button, whenever people wish? When our digital selves are always available, will we ever become part of the past?