If you've already posted an activity, you can add photos to the activity afterward by following these steps:
Via iPhone or Android
Open the AllTrails app
Tap the 'Activity' tab
Open the specific activity for which you would like to add photos
Scroll down to the 'Photos' section and tap on the '+' icon
Select your photos
Via desktop or laptop computer
Open AllTrails.com
Hover over your profile picture in the top right corner and click 'Show profile'
Click the 'Activities' tab
Click the specific activity for which you would like to add photos
Scroll down to the 'Photos' section
Click the '+' icon
Select your photos
Click 'Upload'