Once the photos page is open, unplug the device (tablet, or phone) from the scouting briefcase. You can go visit all the teams and take pictures of bots while leaving the page open. When you are done, come back and plug back in before pressing the "Upload all" button.
Pressing the button for a team will let you select an image. On phones and tablets there is an option to take a photo right then. There should be no need to take photos ahead of pressing the button.
This scouting system only stores one picture of each robot's side and top every season. If you take a second picture of a bot, it will replace the existing picture. You only need to take a picture when there is none already, or you could take a better picture.
When the bot photos page is opened from an event page, the teams taking part in that event are listed. You can add additional teams using the "Add Team" functionality at the bottom of the page.