Why a service? Seems the images are static coded from 0 to 100 and could easily just be a CDN reference rather then burning a service on every hit? URL/progress/1.png .. 100.png done.. :)
If you want flexibility for future you can use a CDN path that includes options you want and later could switch to code the makes the requested image and caches result, returning the cached result.
I imagine the concept is to code the service for fun but I have to wonder to myself when something like this is shared but no concept of scale is considered. I know premature optimization is the root of all evil but isn't building a service to serve a static image basically premature optimization that "maybe some day I'll add more dynamic features".. :-)
;-)