Good day
There is no way to combine multiple observations into 1.
What you can do:
Open up all the observations you want to combine. Select the primary one and delete the rest.
Open the folder on your PC with the photos in. Then Drag/Drop them onto your primary observation photo. You will see a little scrollie at the bottom below showing you it loading the pic. Do this with all the pics you want to add…
Hope this helps.
Not sure how to do it on a phone. Someone else can answer there.