I think the biggest potential differences in the next 10-20 years will come from the fact that most photographs taken by consumers now are taken by smartphones. Small changes to software and hardware capabilities may add up, or there may be a major technological change that the entire market adopts that causes a sea change. It could be something as simple as fitting significantly bigger sensors, so depth of field and low light sensitivity are drastically changed and every photo looks like it was taken by a DSLR, or something big like the maturity and adoption of light-field cameras that allow you to refocus and make 3D images. If there was a universal file format created, and adopted by Apple in iOS, that supported light-field info and/or the distance to every pixel, and allowed native support for all the resulting features in every browser and app, clicking on an "old" picture and not being able to refocus it or see it in 3D will make it clear that it's an "old" one.
And there is probably something much more noticeable that we just aren't thinking of.