Even though this topic is old I'll post my own personal tutorial on how to enable thumbnails. My tutorial might be easier to follow to some.
Enable Thumbnails in Windows Explorer
Step 1. Click on
Start
Step 2: Right-click on Computer and choose
Explore
Step 3: In the upper left corner of the window, click on the
Organize drop down menu and choose
Folder and Search Options 
Step 4: Click on the
View tab and locate the setting called
Always show icons, never thumbnails. Uncheck the box.

Step 5: Click
Apply 
Step 6: Now click on
OK to close the window to complete the process.

An example of a thumbnail is shown above.
That's it!To change the visual appearance of the thumbnail, navigate to the folder that contains images of the thumbnails you wish to see. Click on the drop down arrow next to
Views.

From here you will be able to change the size or appearance of the thumbnail itself by selecting
Extra Large Icons, Large Icons, Medium Icons, or Small Icons.
