Prepare lossless images (screenshots, for example) for being posted to Twitter, scaling them reasonably and making sure Twitter will keep them lossless.

twittify.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from __future__ import print_function
from __future__ import unicode_literals

"""Turn a lossless image into something in a resolution suitable for Twitter,
and turn down the opacity on a single pixel so Twitter won't convert it.
"""

import glob
import os
import sys
from argparse import ArgumentParser
from PIL import Image

def twittify(image, scale=1):
    color_mode = image.palette.mode if image.mode == 'P' else image.mode
    has_alpha_channel = color_mode in ('RGBA', 'LA', 'RGBa') or 'transparency' in image.info
    image = image.convert('RGBA')
    
    if scale != 1:
        image = image.resize((image.width * scale, image.height * scale))

    if has_alpha_channel:
        has_transparency = next(True for p in image.getdata() if p[3] != 0xFF, False)
    else:
        has_transparency = False

    if not has_transparency:
        bottom_right = (image.width - 1, image.height - 1)
        # Setting an even slightly translucent opacity on even a single pixel
        # makes Twitter keep the image as a lossless PNG.
        bottom_right_pixel = image.getpixel(bottom_right)
        bottom_right_pixel = bottom_right_pixel[:3] + (254,)
        image.putpixel(bottom_right, bottom_right_pixel)

    return image

def main(argv):
    script_name = os.path.basename(__file__)
    parser = ArgumentParser(prog=script_name, add_help=False)
    parser.add_argument('-h', '--help', action='help', help="Show this help and exit.")
    parser.add_argument('-s', '--scale', metavar="scale", default="1", type=int,
                        help="Scale the image to [scale] times the size.")
    parser.add_argument('files', metavar='file', nargs='+',
                        help="An image file to twittify.")
    args = parser.parse_args(argv)
    
    in_paths = []
    for path in args.files:
        in_paths += glob.glob(path)

    scale = args.scale
    
    exit_status = 0

    for in_path in in_paths:
        out_path = os.path.splitext(in_path)[0] + '-twitter.png'
        try:
            image = Image.open(in_path)
        except IOError:
            print("Error: couldn't open {}.".format(in_path), file=sys.stderr)
            exit_status = 1
            continue
        image = twittify(image, scale)
        try:
            image.save(out_path)
        except IOError:
            print("Error: couldn't save to {}.".format(out_path), file=sys.stderr)
            exit_status = 1
            continue
        
        print("Twittified {} to {}.".format(in_path, out_path))
    
    return exit_status

if __name__ == '__main__':
    sys.exit(main(sys.argv[1:]))