The fact that some users are getting the updates and not others, means that the server is most likely sending the new content, but the 'other' users' browsers are probably caching the content. Which means, there's nothing YOU can do about specifically.
Though you can TRY to send these headers (which may help the browser to not cache the content) (in PHP)
<?php header("Cache-Control: no-cache, must-revalidate"); header("Pragma: no-cache"); ?>
"Is there a way to force updating if I made changes to any of these files?"
- You can try using dynamic filenames, such as 'image003_[timestamp].jpg'