HELP@!!@ Accidently declined to rebuild thumbnails in iPhoto!!!!

I opened iPhoto and a message popped up that said something to the effect of "for best performance rebuild your thumbnail cache" there were two options: rebuild now and ok. I accidentally clicked ok and the message went away. I tried quitting iPhoto and re-opening it to get the message to pop up again and it did not, I also tried restarting my computer. Does anyone know how to manually rebuild the thumbnails or if there is somewhere in the toolbar to do so?
any help would rock my world
thanks