Picture, Photo, Image Thumbnail not showing up in Windows 7

                                ----Method 1----

1. Open your Windows Explorer.

2. From the menu, Organize > Folder and Search Options

3. This will pop-up the Folder Options  window.

4. Click the View tab.

5. Uncheck the following option, if it’s checked.

Always show icons, never thumbnails

6. Click on Apply.

7. Click on OK.

                                       ----Method 2----

1. Start > Control Panel > System > Advanced system settings.

2. In this window, select Advanced tab.

3. In this tab, select Performance > Settings > Visual Effects

4. Select the check box “Show thumbnails instead of icons” and click on Apply button.

Method 3

1. Open the folder in Windows Explorer .

2. Click the down arrow next to Views at the top. 

3. Choose Medium icons and see if the thumbnails start working again.

source: http://techlikes.com/2011/01/01/picture-thumbnail-previews-windows-7.html