To update (flash) a recovery image onto your phone allows you to update or replace your recovery environment on your phone. There are a number of ways to flash a new recovery image, some of the more commonly used methods are using a tool such as ROM Manager or using development tools such as Fastboot. There is also a third method using a standalone utility "flash_image" which allows the flashing of recovery using the terminal emulator on the phone.
flash_image is not a new tool, it has been used on Android since the beginning, many custom ROMs include the utility as part of the ROM itself though not all stock ROMs (including the stock ROM on the G2) include it.
1. A permanently rooted (with S-OFF) phone
2. The flash_image binary
3, A recovery image that is compatible with your phone and ROM
3. A terminal emulator application on the phone or use of an ADB shell
1. Get the flash_image binary and recovery image onto your phone
2. Copy or move the flash image binary to your system and make executable
3. Use flash_image to update your recovery environment
Stage 1: Get flash_image and recovery image on your phone
1. Download the flash_image binary (not needed if you are running a custom ROM that includes this binary)
2. Download the recovery image you wish to use
You can either directly download the files onto your phone or onto your PC and transfer to your phone. Probably the easiest way to do this is to connect your phone to your computer via USB and mount USB storage, then copy the files to your SD Card. Alternatively you can use ADB Push, bluetooth file transfer or several other methods.
Stage 2: Copy or move the flash image binary to your system and make executable
In terminal emulator:
mount -o remount, rw /system
cd /sdcard (or wherever you downloaded/copied the file)
cp flash_image /system/bin
chmod 777 flash_image
In terminal emulator:
su (not needed if using the same terminal session used in the steps above)
cd /sdcard (or wherever you downloaded/copied the recovery image)
flash_image recovery recovery.img (use the appropriate file name for the image file you are flashing)
Any of the commands that call for using a terminal emulator on the phone should work fine using an ADB shell if you prefer.
This was tested on G2 but I can't think of why it would not work on Desire Z or any other phone for which this version of flash_image works. Obviously different phones have different compatible recovery images.
I've attached a zip file containing the flash_image binary that I extracted from the CM 6.1 update zip. I suspect most custom ROMs already have flash_image.
If you are wondering "Why should I use this method over using ROM Manager?" you could be using a recovery image that ROM manager doesn't support, for example ClockWorkMod Recovery 3.x which is required for some experimental ROMs.
If you are wondering "Why should I use this method over using fastboot?" The two main reasons are you can't use fastboot if you are not with a computer with working ADB and using fastboot requires that you have previously flashed the engineering HBOOT.
This is my first guide so I'm open to suggestions or feedback.