Storage is cheap. Bandwidth is usually the killer.

Chuck it behind Cloudflare and take advantage of the Bandwidth Alliance

https://blog.cloudflare.com/bandwidth-alliance/

If you become popular replace the orange cloud with a Worker. Different TOS and the CEO said it's cool to use workers in this way https://news.ycombinator.com/item?id=20791660

Based on IPs and whois, imgur uses fastly.

If you're a Tier 1, then bandwidth is free by definition.