Adds a solid-colored border to images of a known width/height

png_add_border.py

# you will need a folder called `images` with pngs you want to add a border to,
# and a folder called `altered` where they'll be written to

from PIL import Image
import glob

WIDTH = 200
HEIGHT = 500
MARGIN = 10
COLOR = (255, 255, 255, 255)  # RGBA white

bg_w, bg_h = (WIDTH + MARGIN * 2, HEIGHT + MARGIN * 2)
for fullname in glob.glob('images/*.png'):
    name = fullname.split('/')[1].split('.')[0]
    img = Image.open(fullname).convert('RGBA')
    background = Image.new('RGBA', (bg_w, bg_h), COLOR)
    offset = ((bg_w - WIDTH) // 2, (bg_h - HEIGHT) // 2)
    background.paste(img, offset)
    background.save(f'altered/{name}.png', 'png')