A Pythonista Share Extension script that makes your photo square (instagramifies your photo) by adding black background to the narrower side and saves it to your photo library.

instagramify.py

import appex, photos, sys
from PIL import Image

img = appex.get_image()
box = img.getbbox() # left upper right lower
width = box[2] - box[0]
height = box[3] - box[1]

if width > height:
	size = (width, width)
	new_box = (0, (width - height) // 2, width, width - (width - height) // 2)
elif height >= width:
	size = (height, height)
	new_box = ((height - width) // 2, 0, height - (height - width) // 2, height)
else:
	sys.exit()
	
im = Image.new("RGB", size, "black")
im.paste(img, new_box)
im.save('tmp.png')

try:
	album = [a for a in photos.get_albums() if a.title == 'Instagram'][0]
except IndexError:
	album = photos.create_album('Instagram')

asset = photos.create_image_asset('tmp.png')
album.add_assets([asset])
im.show()