It could happen for different reasons that your image thumbnail previews disappear. This could be annoying, but there are a couple of simple ways to get them back, like changing interaction with Windows without installing any software, or using a simple freeware application.

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Step # 1 - Check to see if thumbnails are disabled in Windows Explorer Settings
First, check to see if thumbnails are disabled. To do so, open any folder and press “ALT”+”T” on the keyboard, then select “Folder Options” in the “Tools” tab.
Keep in mind that in Windows 8 there's no tab called "Tools". You just have to press the Windows key on your keyboard; type "Folder Options" and press enter. The dialog box will appear.
Next, click on the “view” tab. You’ll see many options with a checkbox next to each one. Make sure that the checkbox next to “Always show icons, never thumbnails” is unchecked. If it is checked, be sure to uncheck it. Finally, click ‘ok’ and go back to Windows Explorer to verify whether thumbnails now appear.

Step # 2 - Rebuild the thumbnail cache
You can also try rebuilding the thumbnail cache. This can be easily done from the desktop. Just press ‘Windows” + “R” key to get the run prompt, then type this: “ie4uinit.exe –ClearIconCache”
Now check to see whether the thumbnail appears.

Step # 3 - Use Sage Thumbs
A third option is to use “SageThumbs” a Windows Explorer extension that allows users to preview many image formats. Go to the link: “www.code.google.com/p/sagethumbs/” and download the extension. Then follow the steps to install it. Next check to see if your thumbnails are back. If not, run SageThumbs Options for your OS and select the file types you want to add support to, before clicking ok.

And that’s it!

Result: Congratulations! You’ve just learned How to Fix Windows 7 Image Thumbnails.