You cannot find it mentioned in release notes accompanying the most recent Instagram update which brought out a new Snapchat-like Stories feature, but either that or an earlier update has quietly added a much-needed Drafts feature to the popular mobile photography app.
Called by some fans the best feature that has happened to Instagram in quite a while, Drafts lets you save the photo you're currently editing as a draft.
This is quite useful, if not essential, in case you just spent your time making detailed edits to a photo, but ultimately decided not to post it until later.