Are image only CDN's a sustainable market? Maybe they are, and I'm just ignorant on why websites would choose such a thing over a general CDN.
Surely if someone has enough traffic or load to require a CDN then they would want it for their whole site and not just for image resources?
As another poster highlighted, hosting other people's content is tricky. Its certainly not a business I would rather be in. I bet the management of this SaaS service is going to quickly take over your day job.
If you are just hosting your own content you can do it pretty cheaply using the AWS Serverless Image Handler stack:
https://aws.amazon.com/solutions/implementations/serverless-...
And best of luck in your endeavor!