Are you talking about the recovery image? If so, you don't need to put it on your sd card. Download it on your computer, then put it in the folder that has fastboot. It's in the platform tools folder in the android sdk, unless you moved it. Once you do that, then reboot your phone into fastboot mode and connect to your pc. Make sure USB debugging is checked and the phone is in charge mode only. Then run the command from your computer and reboot your phone.