Root ZTE Blade V6 seems complicated
First of all, make sure that you got the proper "recovery.img" for your device. Invalid header is not a good message.

Then,if you are able to communicate with an Android phone via adb, you can boot it into recovery mode by command "adb reboot recovery".

What I suggest for your situation is(I assume you don't need fastboot oem unlock):


Make sure your PC can see your device by the command: "adb
devices".
Boot into fastboot mode by the command: "adb reboot bootloader".
Flash recovery image to recovery partition: "fastboot flash recovery
recovery.img".
Exit fastboot mode: "fastboot reboot". (Some bootloaders may support fastboot boot recovery.img, I am writing for generic.)
After exiting fastboot mode, wait for Android to boot, then check if
your PC communicates with your device: "adb devices".
Reboot into recovery mode: "adb reboot recovery".
Now, the bootloader is expected to load CWM.


In case you need "fastboot oem unlock"(you will see a warning like "partition flashing is not allowed" at 3rd step above, because of verified boot feature.); under Settings->Development Settings, turn on OEM unlock. Then, run below commands:


adb reboot bootloader
fastboot oem unlock