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.
Install User Role Editor
Please note: this method works in the installed version of WordPress on any operating system. The process may be different in the web version.
In WordPress menu, go to Plugins → Add New.
Type “User Role Editor” in the plugin search box and then click on “Install Now.”
Then click on “Activate”.
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.
Reset User Roles
If confusion has occurred in the roles of users who have accumulated a sufficient number, or if you should perform a cleanup of all roles, a reliable solution is to reset the roles.
To do this, go to the list of installed plugins by clicking on the corresponding button in the menu.
Then go to the “Tools” tab and click on “Reset”.
Confirm the action in the next dialog box.
Pay attention to the warning about changes that will follow this reset. Additional information about this is written here.