Yes, the avatar can be changed by the admin or the group members.
To change an avatar:-
1. Go to Chats on the vertical menu to the left side of the page.
2. Choose the conversation for which you want to change the avatar of.
3. Click on the Group avatar at the top.
4. Upload a picture.
1. Tap on the Chat menu icon on top right side corner.
2. Choose the group conversation on which you want to change the avatar of.
3. Tap on the menu button on the top right corner.
4. Tap on the avatar icon.
5. upload a photo and click save.
3. Tap on Settings on the top right corner.
4. Tap on the avatar icon and click edit.
5.Upload a photo.