Launch Powershell from DDC Studio, or from powershell: asnp Citrix*

Get-BrokerCatalog -Name Nameofyourcatalog

Copy the ProvisioningschemeID and paste into notepad
 
Get-ProvScheme -ProvisioningSchemeUid  ProvisioningschemeIDcopiedfromnotepad

Find MasterImageVM in the output.  This is the associated master image