It's a bit of an old thread (last post more than a decade ago). Things have moved on since then, and these days the simple way of doing what the OP wanted (restore a recovery backup) would be to copy it to a USB drive and connect that to the phone. Thanks to USB-OTG your custom recovery can install a nandroid backup from USB storage, so no need to worry about trying to flash images using fastboot unless you actually want to revert to stock.