Over the 54 days of our hike, we took a portrait photo each morning before setting off on the day's walk and then each afternoon when we'd reached camp -- often capturing the morning's optimism and then the afternoon's pain of the journey on our faces, and the slow change over time.
The music was sourced from jamendo, which is a great site of Creative Commons licensed music.
The theory of averaging images is purely mathematical: take all the pixels at each location an average them. But we humans like to get something a little more recognisable, so a little editing is generally employed. Funnily enough, all that editing was already done because my slides were used in the animation... so all I really had to do was to average them :)
What is somewhat interesting is that there is a discernible difference between the morning and the afternoon averages.