The avatars that are won as part of your leaderboard rewards can be found in your Player Journey screen. You can find them by :

 

1. Tapping on your XP bar at the top left of your screen while in the main menu

2. Tap on 'Change Avatar' in the middle of the screen just above your Player Journey bar

 

Avatars are completely separate to characters or skins which are found in the Armory!