If you are dealing with guest writers, they are most likely set to the contributor role in WordPress. The one problem with the default contributor role is that this doesn’t allow contributors to add images to their posts. There are a lot of WordPress role editing plugins out there, we highly recommend the “User Role Editor” plugin by Vladimir Garagulya. As a plugin author, he is very active and has pushed out a lot of updates. Follow the steps below to install the plugin and grant rights to contributors to add/upload images to their posts.
WordPress – Contributors
In WordPress, go to Plugins > Add New.
Type “User Role Editor” in the plugin search box and then click on “Install Now.”
Then click on “Activate Plugin.”
To open the plugin, navigate to Users > User Role Editor.
In the drop-down box, change to “Contributor” as a selection.
Now select the check box for “upload_files” and click on the “Update” button.
This will allow the users with the Contributor role to now see and use the “Add Media” button in the visual editor. Also, they won’t be able to delete any old image in the media library, only their own.