First, it is proper netiquette for authors to post a profile picture. So, post a good image.
Author Picture Netiquette Book Networks
It follows, we want to know what the author of the books look like. And so, author profile is picture of the author. Given, profile associated with the work. Book networks have author pages that allow them to post pictures in their profile.
Book Networks Author Picture
A picture says a lot about the author. First, proud of their work for one. Also, willing to connect with fans on a personal level for another. It follows, work authenticity.
Author Picture Netiquette
It is proper netiquette for authors to post a profile picture because readers want to know what the author looks like.