imagor

imagor resizes, crops, and converts images on the fly from a URL: request /300x200/smart/your-image.jpg and it returns the thumbnail. It speaks thumbor's URL syntax as a drop-in replacement, but it's written in Go on libvips, which makes it several times faster than the ImageMagick-based options. Apache 2.0, fully free, no cloud tier.

Deployment is one Docker container and you're processing images. Wire in S3 or GCS for loading and caching with environment variables, put a CDN in front, and you've rebuilt the core of Cloudinary's transformation API. Resource needs are modest because libvips streams instead of loading whole images into memory.

Solo and small teams: imagor plus a CDN kills an Imgix or Cloudinary bill for straight transformation work. imgproxy/imgproxy is the comparable alternative with a different URL scheme and a paid pro tier. Teams using Cloudinary's asset management, video pipeline, or DAM features are only replacing one slice here.

The catch: one maintainer carries most of the project. It's stable and actively developed, but there's no company behind it selling support when something breaks at 2am.