There are two categories of photos that you can add to a session. The first if a general session photo (generally not a bird) and the second is a session sighting photo (photo of the bird in the sighting).
Adding photos in Casual sesson mode
Adding photos to sightings while a session is in progress is really simple in the Casual season mode. Simply swipe the screen up to show the list of sightings you have logged and then tap on the plus button next to the sighting you want to add a photo too. Now just add a photo to the sighting and you are done.
Adding photos in Pro sesson mode
To add photos to sightings while you are recording a session in Pro mode, simply tap the binoculars at the top left of the screen, then find the sighting to which you want to add an image and tap on the plus button. Now just add a photo to the sighting and you are done.
Adding photos via the Post Sesson Screen
If you have already finished your session, you can still add photos to sightings via the are add 'Post Session Screen' before you post your session. To do this simply tab on the light blue Species and Sighting section below the map, then find the sighting to which you want to add an image and tap on the plus button. Now just add a photo to the sighting and you are done.
Adding photos via Birda's web app
If you have already posted your session and you prefer to add your images via your PC or Mac, then head over to the Birda web app, login, find the sighting or session you want to add photos to and upload all your images from there! Here is a step by step guide on how to add photos via Birda's web app.