The service has actually been around since about 2014, currently, we serve about 400 million images a month, using some 6TB of bandwidth. It's pretty manageable since we use a CDN on top of the service, to cache already processed images.

Hoping to do a write-up of the architecture in the future, but the source and deployment setup is available at https://github.com/DMarby/picsum-photos if you're interested