You uploaded an image and instead of that image, you still see the previous one ?
Why is it occuring?
You uploaded the image A and B under the exact same name.
The image A is loaded in your browser's cache and the image B you uploaded right after will not be considered as a new content by your browser due to the same name. Therefore, the image A will remain in cache and will be displayed until the cache will refresh.
How can I fix it?
It's easy as pie : upload your new image with a different name.
It will force your browser to put a 'new file' in cache.