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.. :)
The problem I have with all of these images, is that they never sit centered with the text in the generated HTML. I love the idea, and I even have some different badges in a few of my repos but man is it frustrating to see sometimes haha. I'm probably the only one with this gripe, but it does drive me nuts.