In a press release, Instagram explained why they were moving away from their square photo format:
It turns out that nearly one in five photos or videos people post aren’t in the square format, and we know that it hasn’t been easy to share this type of content on Instagram: friends get cut out of group shots, the subject of your video feels cramped …
Now, when choosing a photo or video, you can tap the format icon to adjust the orientation to portrait or landscape instead of square. Once you share the photo, the full-sized version of it will appear to all of your followers in feed in a beautiful, natural way.
We’re especially excited about what this update means for video on Instagram, which in widescreen can be more cinematic than ever. While we’ve historically had separate filters for photos and for videos, all filters will now work on all types of moments. You can also adjust the intensity of filters on videos, too.
We won’t have to download so many Instagram photo fitting apps just to upload one picture! Whew