Can't delete photos in iPhoto
I recently purchased a MacBook Pro Retina like a month ago. For some reason my iPhoto won't let me delete photos. It is strange that I could delete photos a week ago but now I can't. I am running the latest iPhoto v9.3.2 and Mountain Lion v10.8.1 on my computer. I am not sure what is the problem for this to happen. Would greatly appreciate any help in trying to solve this problem.
iPhoto '11, OS X Mountain Lion (10.8.1)