Supabase Engineer here
There were a lot of primitives we built to ship image resizing - Storage events exposed via webhooks, rate limiting, a queue on top of Postgres, a smart CDN cache, object metadata endpoints, etc. These are already available if you are self hosting Supabase Storage, so that you can integrate your own CDN, listen to storage events, etc. Over the next few months, we will be working on exposing these on the Supabase platform too.
Is image resizing built on top of Cloudflare too? https://developers.cloudflare.com/images/image-resizing/
We use imgproxy[1] for doing the resizing. We utilize open source services for the entire stack, so that you can self-host Supabase.