Hi Ruatha...
I have the A720 IS as well, so I took a look at mine to see if I could answer your question.
It seems that, to unprotect an image, you simply need to take the same steps you took to protect the image.
In the playback mode, go to the image you want to unprotect, and then go to the menu and scroll to the
'protect' function. Activate that function as if you were going to protect the photo, and then you will see
the little orange 'key' icon, at the bottom left of the screen, showing the image is protected. Just press the
'function set' button and you will see that icon disappear, indicating that the image is no longer protected.
To unprotect more photos, just scroll through the previews to the images you wish to unprotect, and remove
the 'key' icon from each one. When you're done, just press the 'menu' button again, to back out of the protect function.
Hope that helps.