If pictures are an aide-memoire, I would doubt people add themselves into the picture to remember the fact that they were actually there. Surely no-one can be that forgetful or stupid? Possibly for the group selfies (a contradiction in itself) they don't trust their memory to tell them who else was there on the occasion of their visit.
Is it instead that they need to show their friends that they were definitely in the city they are bragging about having visited, that the views they have returned with are not merely facsimiles from the travel agents window? Surely their friends will trust them when they show the photos and describe the view (such as they remember it from the brief uncluttered glimpses their eyes received when the camera was not in front of their faces).
As you can tell, I'm not a great taker of selfies. If you are, I'm sorry for being a bit scathing but I really didn't get it. However, someone from a group I worked with last week gave me a brilliant new insight. She is still coming to terms with the fact that she looks completely different, in a better way, to how she did a few years ago and a selfie is a simple celebration of that success. This reminded me of another participant I worked with earlier in the summer who photographed herself halfway down the abseil as a way of recording her achievements.
So maybe sometimes a selfie can record the fact that we trusted ourselves to step out and achieve, whether that was to get all the way to the top of the Citadel on a baking hot day, overcoming our fears in order to abseil, or doing something to change our appearance. Perhaps selfies should actually be encouraged as part of helping people to change and grow!