In order to image your servers, you need to download an ESXi ISO and any vSphere Installation Bundles (VIBs) required to get the servers to a supported version of ESXi. See the BOM section of the VMware Cloud Foundation Release Notes for information about ESXi support.
You can download the ISO and VIBs from My VMware (https://my.vmware.com) to any location on the Windows machine that is connected to the VMware Cloud Builder appliance. Make sure to record the MD5 or SHA-1 checksums. You will need them when you upload the ISO/VIB to the VMware Imaging Appliance service.
If the required version of ESXi does not have an ISO available on My VMware, you can create one. See Create a Custom ISO Image for ESXi. In order to upload a user-created custom ISO for use with the VMware Imaging Appliance, you must provide an MD5 checksum. You can use an MD5 utility to generate a checksum for user-created custom ISOs.