Glide is a wonderfully easy on-demand image manipulation library written in PHP. It’s straightforward API is exposed via HTTP, similar to cloud image processing services like Imgix and Cloudinary. Glide leverages powerful libraries like Intervention Image (for image handling and manipulation) and Flysystem (for file system abstraction).

http://glide.thephpleague.com/

It comes with the ability to secure image URL's using a private signing key. I'm a bit worried about flooding the server but the owner of the package states that

... the cache filesystem + caching in league/flystem (with Redis/Memcache) should (at least this is the case in my testing) result in a speedy response...

There's also a risk that over time your cache will become gigantic.  The maintainers of the package are already discussing possible solutions.