An observation records an encounter with an individual organism at a particular time and location. This includes encounters with signs of organisms like tracks, nests, or things that just died… If you record an observation of a tree, then go back a day later to take another photo, please add a new observation using the new photo, because it represents the tree at a different point in time.
If you’re taking photos on different days, they should be different observations. If they’re taken around the same time on the same day, you’d typically combine the photos into one observation. More on this topic here: https://forum.inaturalist.org/t/what-is-an-observation/3367