Instagram encourages you to 'live in the moment' and because of this it's very difficult to automate the scheduling and sending of Instagram posts, for starters Instagram don't expose a mechanism to send images except through their own official mobile application; you can't even send images from their website.
Our Social Scheduler web-application (available from https://www.socialscheduler.co.uk) enables you to schedule social media messages for Twitter, Facebook, LinkedIn, Pinterest and Instagram. All scheduled posts will automatically be sent to the social platform at the date and time you specify, except Instagram (for the reasons outlined already).
To work around the restriction, this Android app enables you to view scheduled Instagram posts that have not yet been sent to Instagram. The queued Instagram posts are listed for you to review - you simply click on the photo to launch send the photo to the official Instagram app, you then manually paste the text in the appropriate 'caption' field, just before you click publish (the text is automatically copied to the clipboard for you; you simply need to paste it into the Instagram app).
The initial version does not notify you when a post is due to be sent (via push-notifications) - this will be coming soon! All you need to do is load the app periodically to see whether messages need to be sent.