How to force image (cache) update in README.rst on GitHub
There is also a trick that can be useful. It is nothing but just adding a question mark at the end of the image extension.
See the question mark (?) at the end. You can also see the live example here in my GitHub profile.
You can see more about this here.
I believe I've understood how to do this now. Instead of serving a raw png file for example, serve the image through a php file (many examples on SO).
Once you're displaying the image with PHP, add the following headers:
header('Cache-Control: no-cache');
header('Expires: ' . gmdate('D, d M Y H:i:s \G\M\T', time() - 36000));
By setting the no-cache and having the cache expire in the past, I have found that GitHub's CDN updates the images automatically on every refresh.
I had a look at what shields.io does.
It sets this header (Source):
Cache-Control: max-age=2592000
curl -X PURGE {url of cached badge image}