How to Convert JPG to PNG in Python

JPG-to-PNG is a one-line conversion in Pillow. The interesting questions are: why are you doing this? PNG is bigger than JPG (typically 3-10x) and won't recover JPG-lost detail. The valid use cases are adding transparency, preparing for further lossless edits, or matching a downstream API that requires PNG.

Method 1: Pillow (the canonical option)

Pillow is the de facto standard for image work in Python. JPG-to-PNG is two lines.

pip install Pillow

from PIL import Image

def jpg_to_png(in_path: str, out_path: str, optimize: bool = True) -> None:
    with Image.open(in_path) as img:
        # Convert to RGB if image has weird modes (CMYK, palette, etc.)
        if img.mode not in ("RGB", "RGBA", "L"):
            img = img.convert("RGB")
        img.save(out_path, "PNG", optimize=optimize)

jpg_to_png("photo.jpg", "photo.png")

To add transparency (e.g., make white pixels transparent — useful for logos saved as JPG):

from PIL import Image

def jpg_to_png_with_alpha(in_path: str, out_path: str, threshold: int = 240) -> None:
    img = Image.open(in_path).convert("RGBA")
    pixels = img.load()
    width, height = img.size
    for y in range(height):
        for x in range(width):
            r, g, b, _ = pixels[x, y]
            if r > threshold and g > threshold and b > threshold:
                pixels[x, y] = (r, g, b, 0)  # transparent
    img.save(out_path, "PNG", optimize=True)

jpg_to_png_with_alpha("logo.jpg", "logo.png")

For huge images, the per-pixel loop is slow. Use NumPy: arr = np.array(img); mask = (arr[..., :3] > 240).all(axis=-1); arr[mask, 3] = 0.

Method 2: Wand (ImageMagick bindings)

If you already use ImageMagick on the command line, Wand exposes the same engine in Python. Useful for mixed-format batch pipelines.

apt install libmagickwand-dev
pip install Wand

from wand.image import Image

def jpg_to_png(in_path: str, out_path: str) -> None:
    with Image(filename=in_path) as img:
        img.format = "png"
        img.compression_quality = 95  # PNG compression level
        img.save(filename=out_path)

jpg_to_png("photo.jpg", "photo.png")

Wand is overkill for a simple format conversion. It shines when you need ImageMagick-specific features (color management, advanced compositing, perceptual hashing) alongside the format change.

Method 3: ChangeThisFile API (no library)

If you don't want Pillow in your binary, or you're processing JPGs in unusual color spaces (CMYK, Lab) where you don't trust Pillow's auto-conversion, the API does it. Free tier covers 1,000 conversions/month.

import requests

API_KEY = "ctf_sk_your_key_here"

def jpg_to_png(in_path: str, out_path: str) -> None:
    with open(in_path, "rb") as f:
        response = requests.post(
            "https://changethisfile.com/v1/convert",
            headers={"Authorization": f"Bearer {API_KEY}"},
            files={"file": f},
            data={"source": "jpg", "target": "png"},
            timeout=60,
        )
    response.raise_for_status()
    with open(out_path, "wb") as out:
        out.write(response.content)

jpg_to_png("photo.jpg", "photo.png")

The API auto-handles CMYK, grayscale, and unusual JPG variants. Output is sRGB PNG with optimal compression.

When to use each

Production tips

• Don't expect quality recovery. JPG-to-PNG preserves whatever's in the JPG, including compression artifacts. PNG won't make a low-quality JPG look better — it just stops further degradation.
• Use optimize=True for served images. PNG optimization tries multiple compression filters and picks the smallest. Adds CPU time on save; saves bandwidth on every download.
• For batch jobs, parallelize with concurrent.futures. Pillow releases the GIL during encoding/decoding, so threads work fine.
• Strip EXIF if you don't need it. Default Pillow strips EXIF on save anyway; if you copy via img.save(out, exif=img.info['exif']) you keep it. JPG-side GPS metadata in particular often shouldn't be redistributed.
• Watch for CMYK JPGs. Print-prep JPGs can be CMYK. Pillow's save fails or produces wrong colors. Convert to RGB first: img.convert('RGB').

Pillow is the right answer for nearly every Python JPG-to-PNG case. Wand for mixed pipelines. The API for unusual inputs or zero-dependency requirements. Free tier covers 1,000 conversions/month.