You can use the commandline wbadmin to specify what your backing up into the WindowsImageBackup.
See: http://technet.microsoft.com/en-us/library/cc742083.aspx
Specifically,
-include : For Windows°7 and Windows Server 2008 R2 and later, specifies the comma-delimited list of items to include in the backup. You can include multiple files, folders, or volumes. Volume paths can be specified using volume drive letters, volume mount points, or GUID-based volume names. If you use a GUID-based volume name, it should be terminated with a backslash (). You can use the wildcard character (*) in the file name when specifying a path to a file. Should be used only when the -backupTarget parameter is used.