If anyone else gets the error I got, clear your cache.
It appears the image is uploaded under the same URL it was before? In that case, Firefox just defaults to the image stored in its cache, without bothering to check for a newer version. So that's why I was seeing the old one.
You don't need to clear your cache; you just need to update it. The refresh button updates your cache on the page you reloaded. If you think any webpage is out of date due to cache, simply go to said webpage and refresh it (This is what Iodine was talking about when he said to hit F5 as most browsers' refresh button is bound to F5.) Of course, if several websites are out of date, you may as well clear your cache to save the time you would otherwise spend clicking refresh. Just be aware that clearing your cache can
remove important data that you'll have to re-enter later.