http://windows.microsoft.com/en-US/windows-8/iso
Windows 8 Consumer Preview ISO files (.iso) are provided as an alternative to using Windows 8 Consumer Preview Setup. If you are on a PC running Windows and want to install the consumer preview on another partition, another PC, or a virtual machine, we recommend you download Windows 8 Consumer Preview Setup and use the built-in tools for converting an ISO image into installation media, such as a DVD or USB bootable flash drive. You can find additional information, including a list of supported upgrades, in the FAQ.

Note before you download: Windows 8 Consumer Preview is prerelease software that may be substantially modified before it’s commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here. Some product features and functionality may require additional hardware or software. If you decide to go back to your previous operating system, you'll need to reinstall it from the recovery or installation media that came with your PC.