According to: http://instagram.com/developer/endpoints/media/
At this time, uploading via the API is not possible. We made a
conscious choice not to add this for the following reasons:
*) Instagram is about your life on the go – we hope to encourage photos from within the app. However, in the future we may give whitelist access to individual apps on a case by case basis.
*) We want to fight spam & low quality photos. Once we allow uploading from other sources, it's harder to control what comes into the Instagram ecosystem. All this being said, we're working on ways to ensure users have a consistent and high-quality experience on our platform.
This means that if you do this you are probably going to be breaking the service agreement and you'll need to scrape and impersonate forms if it's possible at all.