You can make a boot disk with a live OS such as Linux that can be run off a CD. Copy a disk imaging software on that CD along with the OS, or use a USB flash drive to run it off that bootable OS.
You can then boot off the CD, connect to the remote computer as a network drive, FTP drive, whatever you setup for it.
Start the imaging software and for the location to save the file pick the remote computer.
In a corporate setting it's possible to setup an imaging server using Ghost or a similar program that will allow you to pull an image down to a PC from pretty much anywere where you can connect to that server either on the same network or through VPN.