Adding Guest Author content

I’m trying to add guest author content for single.php and author.php. Where can I find some documentation on coding for including guest author content? Below is my issue.

You can see on this link Single Post that the byline shows both names as expected, but the pictures are not displaying properly. I assigned the user’s photo to the avatar section of guest author. But I cannot find any documentation on how to properly call this picture.

Also, if you go to that user’s author page, none of their content is showing, except for their feed. The guest author description, name and email fields are missing.