When you open files from the internet (web pages, images, etc.), your computer caches them so it won't have to download them from the server the next time you open them. That was somebody's wise idea

a long time ago to make surfing faster.
The AOL browser is particularly bad about holding onto files in the cache (Just one of the many things AOL is bad about).
With Windows, you can force it to download from the server by holding down your Ctrl key while you refresh. I don't know about Mac.