Make sure the Windows Aero theme is enabled. Right click Desktop >
Personalize > Themes > click in the Themes list box and select 'Windows
Aero' > OK
If that does not work, you might need to refresh your Windows Experience
Index (WEI) rating.
1. Click Start
2. In the search box, type Performance Information and Tools, hit enter
3. Click the Update my score link
4. Click Continue on the User Account Control dialog
After that check back under Taskbar Properties to see if the option to
enable Taskbar Thumbnails is enabled.