you can't make the backup directly from bootstrap.
let me try to clear things up.
thee are two applications you need (to do things the easy way) to have a working recovery.
1. bootstrap
2. ROM manager
bootstrap will allow you to reboot into a custom recovery, to do this, download the bootstrap apk from the aforementioned site, then put it on your phone, and install it. when you open the application, it will have two buttons, bootstrap (iirc) and reboot recovery. click the first to load the hijack, then you can click the second one to reboot into recovery.
once you boot into recovery (the black screen with green text, it will say clockwork mod somewhere on it) there will be a menu item that says something like backup/restore, navigate to it with the volume buttons and select it with the camera button.
once you select it, it will have options to backup, or restore. select backup and it will make a backup (image) of your system on your sd card.
later if you need to restore from your backup, boot into recovery (either by using the bootstrap app and selecting reboot recovery, or the power + home button) and navigate to the backup/restore menu and select restore.
it will ask you which backup to restore (it is sorted by date and time) if there is more than one, select the one you want to recover and it will restore your system, then reboot and you should be fine.
you can also install rom manager (you still need to download, install and run the bootstrap apk)
after installing and running rom manager, it will have an option to backup or restore, select that and it will automate the above process for you.
ROM manager is essentially a GUI to clockwork mod, it just makes things easier for you.
i hope this helped. sorry if it's a little rambly, but i'm kind of drunk right now