A cloud image is a flexible disk image with a Linux based OS that can adapt to almost any virtual hardware automatically.
Many Linux distributions provide cloud images. Below you can find a list:
Daily builds of cloud images for Ubuntu can be found here:
On the page above, select the desired Ubuntu version, and then click "current". After that you need to scroll all the way down to go to the downloadable images. For use with Ravello you should select the "*-server-cloudimg-amd64-disk1.img". The images are around 250 MB and can be uploaded directly into Ravello.
Fedora makes cloud images available at the following location:
You should select the 64-bit qcow2 format. It can be uploaded directly into Ravello.
CentOS, unfortunately, does not make cloud images available as of yet (July 2014). That said, there's a lot of good automation tools available to build a cloud image yourself. The recommended way would be use use the "images/centos.sh" script from the Ravello SDK. This script will build a CentOS cloud image fully automatically with the latest updates and stores it as a qcow2 image. The script needs to be run on a Linux host with a recent version of libvirt and virt-builder available.