If your thumbnails are messed up then you need to use Disk Cleanup, which is available in your Windows operating system, no matter the version.
Please go to Start > Run or press Windows Key + R button combination and type: cleanmgr. Select the drive where your thumbnails are located and scroll download to the windows that appears and select Thumbnails then click OK and let Windows refresh your thumbnails. Restart the computer and your thumbnails will return to normal.