adb is not available in the bootloader, try using fastboot instead
for a quick and dirty rundown of flashing your device with a new image using fastboot:
adb reboot bootloader
fastboot devices to verify your device is visible, if not, you're probably running windows, and you have to install some funky drivers.
if you don't see 'unlocked' at the bottom of the screen, use the command
fastboot oem unlock
fastboot flash boot boot.img
fastboot flash system system.img
fastboot flash userdata userdata.img
fastboot flash recovery recovery.img
If you feel super confident you could just:
adb reboot bootloader && fastboot oem unlock && fastboot flash boot boot.img && fastboot flash system system.img && fastboot flash userdata userdata.img && fastboot flash recovery recovery.img && fastboot reboot
PS You should only use the flash commands if the referenced files are present, if not you may or may not brick your phone (haven't tried flashing a non-existent file to the device yet)
Warning: Unlocking bootloader wipes off data in your phone.