image: fix build accidentally deleted one method TBR=rsc
image: fix build
accidentally deleted one method
TBR=rsc

R=rsc
CC=golang-dev
http://codereview.appspot.com/4438090

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang