I am facing some issues with graphics driver, so i want do go back to windows 8 now. i have created a System Image recovery for OS drive in my external Hard disk. Now when i use the option of restore from system image, it is saying that the OS version you are trying doesn't match, [it says for example vista 64-bit means you have to restore from vista 64-bit].. Any solutions..?? 

You should be able to use the diskpart clean command to wipe the 8.1 os drive. Then boot from a repair dvd/usb and restore your image from there.