Here is a quick breakdown of a boot.img
boot.img = zimage (the kernel) + ramdisk.
ona stock sensation or a boot.img taken from an ruu, the ramdisk has a protection flag set (ro.secure=1 in default.prop needs to be changed to ro.secure=0).
The kernel devs here, package their kernel updates in a way that copies the current ramdisk, and then comines them with their compiled zimage to make a boot.img which is then flashed. This method is actually the preferred way, and causes the least amount of issues/errors.
But this method fails with a protected ramdisk, which results in the status 7 error about write_raw failed. So to get around this, you must first flash an insecure boot.img.
Most (if not all) custom roms provide you with an insecure boot.img to begin with.
but to shroten the process, i'm going to link an insecure boot.img for those who don't want to change/install a custom rom.