How to Change Your Picture on Yahoo! Mail ?
Your profile picture on any social media site sets you apart and identifies your email picture shouldn't be any different! You can change your Yahoo Mail picture from within your Account Settings menu on both the mobile app and Yahoo's website.
Using a Mobile Device.
1. Tap the Yahoo Mail app to open Yahoo Mail. If you don't yet have the Yahoo Mail app, you can download it for free from the Apple Store (iOS) or the Google Play Store (Android).
2. Tap on the Menu icon. This is the three horizontal bars to the immediate right of the "Inbox" text.
See Full Steps - How to change a Yahoo profile picture