If there are improvements made to the repo, old cards won't get the update since GitHub will keep the cached version of the card and not fetch a new version for a while.
In the code, it's possible to set the image to expire after a certain amount of time, so when there are new changes, the update will appear on all cards within a shorter amount of time.
// set cache to refresh once per day
$timestamp = gmdate("D, d M Y 23:59:00") . " GMT";
header("Expires: $timestamp");
header("Last-Modified: $timestamp");
header("Pragma: no-cache");
header("Cache-Control: no-cache, must-revalidate");
This or something similar can be placed in the index.php below where it sets the content type.