iPhone: share with instagram
Sorry but you don't really have any control over this. Some sharing panels offer a "Open  In..." option in the bottom-right corner. If you check an application that does offer this option (for example Dropbox) you'll likely find Instagram in that list.

For some reason Apple doesn't provide that "Open In..." option in the Camera or Photos apps.

The Facebook and Flickr options are hard-coded by Apple. They always appear in the sharing list even if you don't have the applications installed.