Have you heard of
IFTTT? It stands for "If this, then that" and it's an automation site that allows you to create recipes (like if it is raining, then notify me via e-mail. or if I update my Facebook status, post it on Twitter).
It's quite easy to set up.
Create an account on IFTTT and connect both your
Instagram, and your
Twitter accounts. Then find the below recipe, that states IF you instagram, then tweet. Turn on this recipe, and make sure to test that it works. Going forward, all of your instagram posts should show up as photos on Twitter!