If it has a UI, it has an "API". Let's use the following example: I want to publish the pic I use in any new blog post I create. Let's assume is Wordpress.
- Create a service that is constantly monitoring your blog via RSS.
- When a new blog post is posted, download the picture.
- (Optional) Use a third party API to apply some overlays and whatnot to your pic.
- Place the photo in a well-known location on your PC or server.
- Configure Chrome (read above) so that you can use the browser as a mobile.
- Using Selenium (or any other of those libraries), simulate the entire process of posting on Instagram.
- Done. You should have it.