Do any open-source standalone restful image servers exist?
I would recommend you this project:https://github.com/imbo/imbo
Its easy to use, stable and used in big projects.
But I am still curious about alternatives.
I was looking for options for a project, and I found those two below. They are not a perfect match to your requirements but seem quite mature. I have no experience with them yet, though.
- https://imageresizing.net/ Essential edition is open source. The more advanced solutions are not.
- http://thumborize.me/ (with associated github) has many interesting features like face detection, new codecs, smart cropping.