Our service provides media upload in the same way you would if you sat behind a phone 24x7x365, or you hired an agency to manage your Instagram on your behalf (who would post for you, like any other agency!).
One alternative that several services have put in place is to help you out by "pushing" the image to your phone at the allocated time (sending you a mobile notification) -- these mobile apps are handy because it means you don't have to email the photos to yourself or remember when to post things, however, they don't really solve the problem, because you still have to log into Instagram yourself and post the image. Certainly an improvement for brands, but a long way from useful (in our humble opinion). But services like this (e.g. Later.com or Hootsuite) are something we can recommend to more budget-conscious customers who are happy to perform this additional work themselves. Typically these services are half the cost of ours, but involve at least twice the work to get things posted!
As we have stated previously, Instagram forbids media uploads via their API, and also forbids companies from reverse engineering what they call their "private API" (i.e. the sequence of internet calls that the app uses to upload images/video).
There are also a range of services who have just reverse engineered the "private API" -- and in many ways, the solution is simpler, easier to maintain 24x7, and consistent (which also means cheaper). However with that comes a risk -- what you are doing is in breach of Instagram's terms, and also (arguably) the law in many countries, which forbids reverse engineering proprietary software.