I am doing something similar but I'm using http://wordpress.com for editing my post and storing files. It gives you 3 GB of storage for pics as well so you don't have to go to another site for that. Embedding YouTube videos are the same. You can create your post in plain text and then just switch to HTML view once you're done and copy and paste​ it into steemit. You can save all your posts privately or post them in wordpress at the same time as steemit if you like.