Ah, I see now. I saw your PM and thought you were talking about a graphical image. Didn't realize it was a disk image.
Not sure if this is still the case, but it should be an ISO image if you downloaded the disc image from the Ubuntu site. I don't think I've ever seen them provide a 12 MB disc image outright on their website, unless you're using the PXE mini-iso image. That one may not include everything you need since it's mean for PXE installations. It should work on most hardware, but your mileage may vary on a virtual machine. What is the speed of your connection that you can't download the standard ISO installation?