Exactly how are you creating the image? Please bear in mind that a Windows 7 image consists of two partitions, the first being hidden and typically 100Mb in size; the second being the C: drive with the bulk of the system files installed. Both are necessary for a successful boot.
Also, what sort of licensing are you using for Win 7? Are you using a volume MAK key or a KMS server?
If you are using a retail key for your Win 7 image, you will always find that installing the image on a different machine will throw errors as the anti-piracy security on Win 7 locks the install to things like the MAC address and the bios serial number, amongst many other parameters. Thus a hardware change is quickly recognised and the license disabled.