Problem: You upload new images to replace the old images. You give them the same exact name, yet when you look at the site in your browser, the old images still appear.

Solution: Both Joomla and all browsers have something called "cache". The cache stores images and some other files to save time when uploading a webpage. You need to clear the cache in Joomla and in your browser and try looking at the site again. Pull down System -> Clear Cache