Easiest Way To Image About 15 Computers Over A Network Over Night?

As the title says, I would like to know what are some ways in which a group of computers on a network can be imaged at the same time over night. The reason this must be done is because in the training room at where I work, each different training session configures the computers differently, so it would be nice to have them all start off from the same point, rather than a new instructor having to learn all of what the previous instructors had done to the computers.I know some ways in how to image multiple computers. A little background on the environment:

About 15 computers connected a cross a network

Must be repeatable, and remain available (no dismantling)

Image will be Win7 32 bit, with applications already on it

Must be able to run, and complete over night

The image must be pushed out from one computer to all of the others

The OS of the computer doing the pushing can either be Win7 or XP