You can take photos using your phone's default camera app and add them to your activity after you finish recording. If your phone's camera app saves location data with those photos, their locations can appear on your activity map as red dots.
If photo locations are not appearing, check that your camera's location permissions are enabled on your device.
iPhone
Apple recommends confirming that Location Services is turned on, then checking location access for the Camera app under your device settings.
To review those settings on iPhone:
Open Settings
Tap Privacy & Security
Tap Location Services
Make sure Location Services is turned on
Scroll down and tap Camera
Choose any option other than Never, such as While Using the App
Learn more in Apple's guide: Turn Location Services and GPS on or off on your iPhone, iPad, or iPod touch.
Android
Google notes that your camera can add location information to photos when location settings are enabled. If photo locations are not appearing, check that your camera app has permission to access location.
To review those settings on Android:
Open your device Settings
Tap Apps
Tap Camera
Tap Permissions
Make sure Location is allowed
Learn more in Google's guide: Change your camera location settings.
Note: Updating these settings should help newly taken photos appear correctly on your activity map, but it will not retroactively fix photos that were taken before location access was enabled.
To add photos to your activity, please see How to add photos to an activity.