This may seem obvious but iPhoto doesn't actually delete a photo when you select it and hit "delete". It moves it to the iPhoto trash can - it's still on your disk (and not in the system trash either - which I think is a little unexpected). Since the mantra for taking good pictures is take a lot of pictures I've been taking 4-5 shots of each Pic_A_Day subject. I had 235 pictures in the iPhoto trash can - all still eating up disk space. The only reason I even noticed was that I've been importing from the memory card with iPhoto but then browsing the iPhoto directories with Photoshop Elements and I realized the "deleted" files were still there.