A family tree with the photos is always better!
To change the photo of a loved one that is displayed on your tree, you must go to your tree and click the checkbox.
A new window will open, click "Edit profile" in the left menu.
You can then modify the information of your family member including their profile picture.
If your loved one has yet profile picture, simply click on "Add a profile photo" (1). You can choose a photo from your computer or tablet.
If your loved one already has a profile picture, click "Edit Profile Picture" (1) below their profile picture, you can then choose the new profile.
You will then have the ability to crop the photo.
Once added, validate.