Pillow

Pillow is the Python library for image manipulation: resize, crop, add text, convert formats, apply filters, generate thumbnails. It's the maintained fork of PIL (Python Imaging Library), which was the standard for 15+ years before it was abandoned. Pillow picked it up and kept it alive.

open source (HPND license, basically do whatever you want). You `pip install Pillow`, import PIL, and start manipulating images. It supports JPEG, PNG, GIF, TIFF, BMP, WebP, and dozens more formats.

Everything is free. No paid tier, no commercial version. It's a Python package: you install it and use it.

The catch: Pillow is great for basic to moderate image processing but it's CPU-bound and single-threaded. If you're processing thousands of images or need real-time manipulation, look at OpenCV (faster, C++ backed) or libvips (dramatically less memory usage). Pillow loads entire images into memory, which kills you on large files. For a web app generating thumbnails or watermarking uploads, Pillow is perfect. For a pipeline processing 10,000 high-res photos, you'll want something faster.