What it means is, your computer doesn't download the images for a website every time you browse to it, only the first time. It caches them, so if you go back to the page it doesn't need to download them again.
Then, when the page changes, it doesn't necessarily change what you see in your browser because its pulling a local version, not the version on the site.
To remedy that, you either need to wait until you cache purges on its own, which can take days/weeks depending on your settings, or clear it yourself.
If you google "How to clear cache for XXXXXXX" where the X's are replace it with the browser you are using, you should get about a million webpages with clear instructions.
Good luck!