[dnanian]

No, you're absolutely right: there's always a point where you update the backup, but you've had it around for a longer time period, during which you could have recovered. (In other words, if you haven't found anything during that week/month you need, it's unlikely you'll need that particular checkpoint and can move the bar forward.)

A Disk Image is just that -- a disk image. It has a volume in it, of course. But you cannot boot from an image: you can only boot from physical drives stored on supported boot devices. Images, though, can be booted from once restored to a supported boot device.