All browsers have a built in cache, which is simply a place the browser stores images and files you access often to save time and transfer data. Many times when you upload new images, if the file name is the same, the browser will still pull the file from the cache, know knowing the image has changed. You can quickly clear your browser cache by clicking CTRL + F5 for Windows, APPLE + R keys for Mac/Apple, F5 for Linux) Another solution is to right click the image, open in a new window and CTRL + F5 a few times. If it is a background image, in Firefox you can right click and view the background image, hard refresh a few times and your new image should appear.
In addition, under General Settings > Meta, if you have the Cache Control Header enabled, you may need to disable that before you can view your newly uploaded image(s), then refresh. Search engines look at your meta tags and give you credit if you have the Cache Control Header enabled as it further reduces load time, but this can also prevent you from viewing changes you make to the site, even simple text even after a refresh. It is also fine to just wait a couple days when the cache refreshes on it's own.