The safest way to manage content from outside of the WordPress dashboard, and make use of the WordPress roles and capabilities model, is via XML-RPC.
The basic methods built in to WordPress XML-RPC allow you to add, edit and delete posts and media, and restrict the ability to do so based on WordPress user permissions. The problem is that it’s a little complicated to use if you’re new to programming, and the built-in XML-RPC methods are limited in terms of functionality.
For example, creating a new post with four images, and making one of those images the featured image, and adding different taxonomy terms to the post (categories, tags, etc.) based on some arbitrary criteria can require either making several XML-RPC calls in a certain order, or the need to create a custom XML-RPC method.
Here’s a fairly basic primer on WordPress XML-RPC using the Incutio XML-RPC library, which is what WordPress core uses to handle XML-RPC requests: http://jumping-duck.com/tutorial/using-xml-rpc-in-wordpress/