I used a freeware tool for this, can't remember the one I used but I'll find it when I'm on my work laptop tomorrow. However, I just found another freeware tool (for personal use) for Windows users, TSR Watermark
which, after a quick test now, does the job pretty well.
I actually took this a step further and double stamped my images with name/phone no at the top and the slide to wake at the bottom, as the Kindle doesn't allow anyone who finds a password protected device to access the personal info making the personal info kind of a pointless feature unless you leave your Kindle non-password protected, which seems a bit dumb to me. Just a suggestion for anyone looking to stamp their own images.