VMWare has a number of Virtual Machines (what you call "images") that you can download.

Their Virtual Machine Center at http://www.vmware.com/vmtn/vm/ has the following downloadable Virtual Machines:

SUSE Linux Enterprise Server
Novell Linux Desktop
Red Hat Enterprise Linux
Ubuntu

On their page for Community Virtual Machines, which are VMs put together and uploaded by users, they have:

3 versions of Debian
4 versions of Fedora Core 4
Damn Small Linux
Free BSD
Kanotix
SUSE 10.0 w/KDE 3.5
Kubuntu (Ubuntu with KDE)
Minix
OpenBSD
PocketLinux
2 versions of PC-BSD
Puppy Linux

The have the following pre-releases:

5 versions of Fedora Core 5
FreeBSD 6.0
ReactOS 0.29
2 versions of Syllable

They have several other VMs there as well and more are added all the time.

As mentioned in another post, you can also roll your own VM using any LiveCD distro's .ISO.