The popular third-party Instagram app for Windows Phone, 6tag, has gained support for 1080x1080 image uploads in version 5.3. Recently, Instagram announced that it would begin displaying higher resolution images in its iPhone and Android apps, with no mention of Windows Phone. Developer Rudy Huyn has already successfully added support, and the update is now available.
If you have not already done so, be sure to grab the update from the Windows Phone Store. If you have yet to give 6tag a try, you download it for free from the link below.