Android/iPhone: Instagram has rolled out an update to its app that allows you post landscape or portrait photos and videos. This means you're no longer required to crop your photos into squares.
After you select a photo, pinch the photo or tap the new format button on the left to post a landscape or portrait photo. It's as easy as that. Once you share that photo, it will appear in others feeds as the full image. It seems like the updates are still rolling out, so if it's not available right away be sure to check back later.