A common technique here is to use the query string to control this, with something like ?t=‹timestamp› or ?v=‹version› or ?‹hash of file contents›. Then the server can include proper cache-control headers to say “this is immutable, never going to change, don’t bother asking me if it’s changed”.

A cachebust="yes" attribute wouldn’t be useful in practice because the server would either be serving it to everyone, effectively disabling caching (and which would be better done by actually disabling caching with the cache-control header), or need to decide who to serve the attribute to, in which case there are better solutions. In short, cache busting only works if you have some sort of cache key, which is what the query string technique is all about doing.

What I have done to accomplish this in the past is some filename shenaniganry with the file's last-modified timestamp:

    <img src="/path/to/file.[last modified timestamp].png">

PHP code for this kind of thing might look like this:

    function auto_version($urlpath) {

        // get a filesystem path
        $fspath = $_SERVER['DOCUMENT_ROOT'] . $urlpath;

        // if file doesn't exist, don't mess with the url
        if (!file_exists($fspath)) {
            return $urlpath;
        }

        // insert last modified timestamp
        $mtime = filemtime($fspath);
        return preg_replace('{\\.([^./]+)$}', ".$mtime.\$1", $urlpath);
    }

Then in .htaccess or equivalent to strip the timestamp back off before serving the file:

    RewriteRule ^(.*)\.[\d]{10}\.(css|js|png)$ $1.$2 [L] 

Whenever the file is updated you automatically start serving a new link which points to the same file, and otherwise you can rely on the browser caching. You don't need to mess around with ?params, which also helps semantically because those incorrectly suggest that the resource is dynamic.

How would the browser know if the image changed?

Well, I would prefer an attribute, that is already there for some elements (like css or js files):

  integrity="<hash>"

This would kill two birds with one stone: If it changes in HTML, this means:

- Please bust cache

- Refetch and ensure, that the fetched file is securely valid

And it could be optional for every fetchable resource (img, video, css, js, etc.)

Why not just give it a new url?

Huh? That's what HEAD requests and correct HTTP headers are for, surely?