You can add a thumbnail and a featured image for every author. These pictures need to be uploaded in the media archive of WordPress.
- Go in Media->Add new
- Upload a squared thumbnail 250×250 and a fullscreen image 2500px width
- Go in edit for the single files to obtain the media URL
- Copy the Url, then go in Users and edit the user who needs the picture, pasting the picture URL in the required fields: