I'm using OS 10.6.8 and iPhoto '09 (version 8.1.2), so I'm not sure if this applies to you or not.
First, when you delete a photo from iPhoto, make sure under the left window column Library, you're on Photos and not Events, Faces, or Places. Otherwise, you're just deleting the photo link from the Event, Face, or Place.
Second, you need to empty the Trash in iPhoto by control clicking on the Trash icon in the iPhoto Window.
Third, you may need to to empty the System Trash from the Dock.
Fourth and definitely as an last alternative do this (in OS 10.6.8, not sure of any other OS):
Macintosh HD> Users> youruserID> Pictures> control click iPhoto Library> Show Package Contents> Originals> Year> Event to find the pictures you want to delete.
Good luck.