Image Compression and Resizing with Python and Pillow

Image Compression and Resizing with Python and Pillow

Optimizing images is critical for web performance and efficient storage. Python with Pillow gives you full control over quality, dimensions, and format.

Installation

pip install Pillow

Resize While Preserving Aspect Ratio

from PIL import Image

def resize_image(input_path, max_width, output_path):
    with Image.open(input_path) as img:
        ratio = max_width / img.width
        new_height = int(img.height * ratio)
        img.resize((max_width, new_height), Image.LANCZOS).save(output_path)
        print(f"{img.width}x{img.height} -> {max_width}x{new_height}")

resize_image("photo.jpg", 800, "photo_800px.jpg")

Compress JPG with Quality Control

from PIL import Image
import os

def compress_jpg(input_path, output_path, quality=80):
    with Image.open(input_path) as img:
        if img.mode in ('RGBA', 'P', 'LA'):
            img = img.convert('RGB')
        img.save(output_path, format='JPEG', quality=quality, optimize=True, progressive=True)

    kb_in  = os.path.getsize(input_path) // 1024
    kb_out = os.path.getsize(output_path) // 1024
    print(f"{kb_in} KB -> {kb_out} KB ({100 - kb_out*100//kb_in:.0f}% smaller)")

compress_jpg("original.jpg", "compressed.jpg", quality=80)

Convert to WebP

from PIL import Image
import os

def to_webp(input_path, output_path=None, quality=80, lossless=False):
    if output_path is None:
        output_path = os.path.splitext(input_path)[0] + ".webp"
    with Image.open(input_path) as img:
        img.save(output_path, format='WEBP', quality=quality, lossless=lossless, method=6)
    kb_in  = os.path.getsize(input_path) // 1024
    kb_out = os.path.getsize(output_path) // 1024
    print(f"{input_path}: {kb_in} KB -> {kb_out} KB")

to_webp("photo.jpg")
# PNG with transparency -> WebP
with Image.open("logo.png") as img:
    img.save("logo.webp", format="WEBP", quality=80)

JPG Quality Comparison

from PIL import Image
import os

img = Image.open("original.jpg")
for quality in [95, 85, 75, 60, 40]:
    path = f"quality_{quality}.jpg"
    img.save(path, format='JPEG', quality=quality, optimize=True)
    kb = os.path.getsize(path) // 1024
    print(f"Quality {quality:3d}: {kb:4d} KB")
img.close()

Typical for a 2 MB original:

Quality  95: 1850 KB
Quality  85:  980 KB  <- recommended balance
Quality  75:  640 KB
Quality  60:  380 KB
Quality  40:  210 KB  (visible degradation)

Batch Processing

from PIL import Image
import os

def batch_compress(input_folder, output_folder, max_width=1920, quality=85, fmt="WEBP"):
    extensions = {'.jpg', '.jpeg', '.png', '.bmp', '.tiff', '.webp'}
    os.makedirs(output_folder, exist_ok=True)

    files = [f for f in os.listdir(input_folder)
             if os.path.splitext(f)[1].lower() in extensions]
    print(f"Processing {len(files)} images...")

    for filename in files:
        path_in  = os.path.join(input_folder, filename)
        out_name = os.path.splitext(filename)[0] + '.' + fmt.lower()
        path_out = os.path.join(output_folder, out_name)
        try:
            with Image.open(path_in) as img:
                if img.width > max_width:
                    ratio = max_width / img.width
                    img = img.resize((max_width, int(img.height * ratio)), Image.LANCZOS)
                if fmt in ('JPEG', 'JPG') and img.mode in ('RGBA', 'P'):
                    img = img.convert('RGB')
                img.save(path_out, format=fmt, quality=quality, optimize=True)
            kb_in  = os.path.getsize(path_in) // 1024
            kb_out = os.path.getsize(path_out) // 1024
            print(f"  OK {filename} ({kb_in} KB) -> {out_name} ({kb_out} KB)")
        except Exception as e:
            print(f"  ERROR {filename}: {e}")

batch_compress("original_photos/", "web_photos/", fmt="WEBP")

Square Thumbnail

from PIL import Image

def square_thumbnail(input_path, output_path, size=400):
    with Image.open(input_path) as img:
        img.thumbnail((size * 2, size * 2), Image.LANCZOS)
        canvas = Image.new('RGB', (size, size), (255, 255, 255))
        x = (size - img.width) // 2
        y = (size - img.height) // 2
        canvas.paste(img, (x, y))
        canvas.save(output_path, quality=85)

square_thumbnail("product.jpg", "thumbnail.jpg", 400)

Add Watermark

from PIL import Image, ImageDraw, ImageFont

def add_watermark(input_path, output_path, text="(c) My Brand"):
    with Image.open(input_path).convert('RGBA') as img:
        overlay = Image.new('RGBA', img.size, (0, 0, 0, 0))
        draw = ImageDraw.Draw(overlay)
        try:
            font = ImageFont.truetype("arial.ttf", 36)
        except:
            font = ImageFont.load_default()
        bbox = draw.textbbox((0, 0), text, font=font)
        x = img.width  - (bbox[2] - bbox[0]) - 20
        y = img.height - (bbox[3] - bbox[1]) - 20
        draw.text((x, y), text, fill=(255, 255, 255, 180), font=font)
        Image.alpha_composite(img, overlay).convert('RGB').save(output_path, quality=90)

add_watermark("photo.jpg", "photo_watermarked.jpg")

Format Comparison

Online Optimization

To compress images without installing Python, KaijuConverter converts between JPG, PNG, WebP and more directly in your browser.

Related conversions

Most teams that read this guide convert images in one of these directions:

• JPG to WEBP
• PNG to JPG
• HEIC to JPG
• PNG to WEBP
• TIFF to PNG
• RAW to JPG