Depending on how you wish to install your image (and how it's being made) you'll want to acquaint yourself with:
NetBoot. Requires a computer running Mac OS X Server, or a Client modified to have NetBoot enabled (instructions - main article for 10.3/10.4, see comments for 10.5)
You will also need some way to image your devices, it's recommended to use one of the following applications to actually image the computers themselves.
DeployStudio. This is the most popular and the imaging suite I personally use. It allows you to deploy a monolithic image, or even deploy a base image and then add packages after it has been installed. (eg.
NetRestore. While NetRestore is not in active development by Mike Bombich anymore it has been rolled into Mac OS X Server 10.6 for the future and the current version still works well. This is excellent for releasing a base or monolithic image
Casper Imaging Suite. This is the only commercial imaging suite in this list; I had the pleasure of beta testing this product and it's very, very nice.
For making your image you may also want to consider instaDMG which allows you to make a brand new, clean image from scratch that has never been started up instead of having one computer you set everything up on and then have to clean before sending out. All 3 deployment applications (DeployStudio, NetRestore, Casper Imaging Suite) will also allow you to make a "master" image from an existing hard drive if you want to go that route.
Additional Resources for NetBoot: NetBoot across subnets, Troubleshooting NetBoot
For the full documentation of how the System Imaging tools (and NetBoot) can be set up in Mac OS X Server: PDF Documentation
Lastly, here are some additional questions on Server Fault that have talked about imaging on Macs: