/sp is the switch to specify the partition. Straight from the manual -
/sp
Use this option to specify an individual partition ID to restore from a full backup. p is the source partition ID (in hex or decimal notation).
Default if omitted: If you supply a backup of an entire hard drive as the source for restoring, Image for Windows will restore all partitions contained in the backup.
here is an example:
I:\imagew.exe /r /o /fd /sp:1 /d:0 /f:"I:\testimage.TBI" /rb:0