I built a simple static web app (GitShare) where you paste any public repo URL, customize it live and then export as PNG/JPEG/WebP. It runs entirely in your browser (it uses GitHub's API client-side, so you're rate limited to 60 unauthenticated requests per hour).
Tested on Chrome, Firefox, Safari, Brave, and DuckDuckGo on macOS. Fixed tons of bugs, and I think it's reliable now. But if you hit issues on any of those or another browser/OS please let me know, happy to fix them ASAP (and it's open to feature requests and contributions, of course!)