Regarding stub

I'm sorry, it's not clear to me what your specific question is. Let me see if I can explain how this would work, assuming your Author content type contains a node reference field named field_picture. Your picture migration class should define a createStub() method, that creates a stub picture node and returns its ID in an array. Then, your author migration class would have a mapping like:

// 'picture_id' is whatever field references the picture in the JSON source
$this->addFieldMapping('field_picture', 'picture_id')
     ->sourceMigration('picture migration machine name');

However, I'm not seeing why you would need to deal with stubs in this use case - if you simply run the picture migration, then the author migration, it should just work without stubs, you only need stubs when there's a chicken-and-egg problem. Unless... do the pictures have a reference back to the authors? In that case it does make sense - so, with my example, you'd also have in the picture migration:

$this->addFieldMapping('field_author', 'author_id')
     ->sourceMigration('author migration machine name');

So, with createStub defined in the picture migration class, you would run the author migration first, which would cause stubs to be created for the referenced pictures, then when you run the picture migration it should fill in the stubs.

Does this address your question?

How precisely are the authors and pictures related? If I understand your original post correctly, in Drupal you have an Author content type, which contains a node_reference field that references nodes of type Picture. So, presumably in your author JSON there is a reference to the related picture - in your Author migration you would map that reference to the node reference field, with a sourceMigration() pointing to your picture migration. You run the picture migrations first, then the author migrations, and it all works out.

Now, if I understand comment #2 correctly, the pictures may come in at some point days after the authors? Thus, the original author JSON did not originally have a reference to the picture, but now it does? In that case, you would need to rerun the migration with --update to pick up the changes. Still no stubs involved or necessary.