I don't know if this is the best practice, but we added an image with a snow delay for our school. For some users the image wouldn't come up. Is this a matter of the browser or something else? I had to use Ctrl + F5 to clear the cache to see the updated image.
It's definitely a caching issue. There are a few ways to correct thisThe simplest way is to append a unique value as a query string to the name of the image so it always appear to be new to the browser and it requests the image evrey time. Using a timestamp is the easiest way to do this.
You also serve that image through a PHP (or any server side language file) and send no cache headers along with that image.
You can also place the image in its own directory with other images you don't want cached and place this .htaccess file in there to prevent caching.