I am trying to create a USB drive that I can use to install a custom image.

I've created my final image as a .dmg using disk utility, it works, that's not the issue. Where I'm having the problem is I want to be able to put my .dmg on a USB drive, set that as the startup disk, reboot the system and have it erase the hd and install my custom image (or instead of setting as the startup disk, access the USB drive by holding the Option key on startup...which would be preferable for new out of the box machines).

Background...I work for a school district, we have a bunch of new MacBooks that all need the same image, in the past I would have just booted into target mode and connected via firewire to another machine to restore the drive with disk utility but...no firewire port. I don't want to use the install DVD to do this on every machine because it seems like a waste of time.

Additionally I do have some USB drives that will do this, I just replace the .dmg that's in a Configurations folder with mine and it does exactly what I want. I didn't make the drives though, they're issued to us through a apple/education program in my state. So basically I want to know what they did to make these drives work the way they do...yes, I could just use theirs, but it feels like cheating and I'm curious so I want to know how to do it myself.

I know I can't be the only person in the world who wants to do this but I cannot seem to find a solution. If there is a thread that addresses this somewhere please point me in that direction. I feel like there's probably something simple that I'm missing but...I'm still missing it so...anybody???