1. Open Regedit and go to:
2. In right-side pane, create a new DWORD (32-bit) value ExtendedUIHoverTime.
3. Open it, select Decimal base and give it a value of 10000, which means 10 second delay.
4. Log-off and log-on back again to see the change.
Unfortunately, it's not possible to completely turn off Taskbar thumbnails in Windows 7, as the functionality has been removed by Microsoft.
Yeah, not the best solution, but it works quite well.