On many computer systems, the system will store initial images in as a "thumbnail cache."
If your files have the same name, the computer will show the initial ‘cached’ image that it stored.
You may need to erase the ‘thumbnail cache’ from your system in order to see thumbnails of the actual photos instead of previously ‘cached’ photos.
One easy way to clean out the old cache is to use CCleaner Free. You can download a free version of CCleaner from CCleaner.com. After installing CCleaner, you can use it to ‘clean’ your Thumbnail Cache.