Within Social > Posts, you can add permissions for Media Attachments. This permission determines if specific users are allowed to attach media (i.e., images and videos) from their local computer or a public URL when creating social posts.
If the Allow File Upload checkbox is not selected for a role, local upload and public URL options are hidden from the post composer entirely, and users can only select media from the organization's approved Asset Library.
If the Allow File Upload checkbox is selected then users can uplaod files from their local computer. The permission is selected by default (file upload allowed) for all existing roles; you must explicitly uncheck it for roles you wish to restrict.