Uplet

Uplet is a program that lets you upload photos to Instagram directly from your Mac. You don't need to use your smartphone or take a photo on the spot. Just select a photo from your computer's memory and post it.

The program's interface is as simple as it can get. You just have to drag a photo to the small square you see. Then just write the text you want below it, as if you were using the official Instagram app. Easy peasy.