Do you have the idea that you want to upload images to your server by SFTP and then have access to the images through the WordPress media manager, it will not work you will run into a small problem with the WordPress media manager in that it will not find your images on the server.
The images that you load to your server with SFTP can not be seen by WordPress media manager, as an image have to be register with WordPress.
Two files that I uploaded to the WordPress folder
WordPress media manager when you upload images adds meta-data to the database. More details can be found in these two articles – https://codex.wordpress.org/Using_Image_and_File_Attachments
You will notice that only one file is shown in the media
And you can see that in the database that image is shown, not the other one.
Now everything is not lost if you upload directly to your server there is a plugin that you can use to simply import those files from the server directly into WordPress.