"image" is the working title of a kernel virtualization project first discussed at EuroBSDCon2006DevSummit in Milan. The terminology used nowadays is VIMAGE for the subsystem virtualization framework and VNET for the virtual network stack.
Notes from devsummits
Notes from the EuroBSDCon devsummit 2006: /NotesEuroBSDConDevsummit2006
Notes from the EuroBSDCon hackathon 2007: /NotesEuroBSDConHackathon2007
Notes from the BSDCan devsummit 2008: see http://people.freebsd.org/~bz/200805DevSummit/
Notes from the Developer Summit 200808: /Notes200808DevSummit
Notes from the Developer Summit 200810: /Notes200810DevSummit
Notes from the Developer Summit 200905: /Notes200905DevSummit
Basic FAQ-like documentation on the vimage virtualization code, if you want to get an overview, can be found here: /BeginnersGuideFAQ
VImage kernel options during the transition and for the final product /KernelOptions
An unsorted braindump of a /TODO list.
Sample shell commands on how to create vnets /VNETSamples
Contributions on how to run Linux in Jails
http://p4web.freebsd.org/@md=d&cd=//&c=lgB@//depot/projects/vimage/?ac=83 Perforce development branch with some documentation
http://imunes.tel.fer.hr/imunes/ IMUNES - a scalable network topology emulator
http://cs.itd.nrl.navy.mil/work/core/ Common Open Research Emulator (CORE)