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.