I'd like to use custom icons to better distinguish machines running the same OS from each other, in VirtualBox. is this possible? If so, what file(s) do I need to add/edit?

Examples:

I've got two Windows 7 VMs. One I use as a sandbox for testing various things, and the other I use for when I need to connect to work (ideally, my personal system - the host machine - never directly connects). I'd like to have perhaps a beaker for the sandbox, and a suitcase for the work machine.

I've got two Ubuntu VMs. One is BackTrack Linux, the other is a build I'm using to learn more about the OS. I wouldn't mind keeping the regular icon for the latter, but I'd like to use one of BackTrack's icons or images for the former.

I'm running VirtualBox 4.1.6 on Windows 7 x64.