Today cameras take really big photos and not matter how big your hard disk is, sometime you will have it full, so it is a good practice to delete some of the photos you do not want after importing them to iPhoto.
But if you just select them and press the delete button, they will be sent to the Library trash, and will disappear from your library but will still be using hard disk space.
So, here is a small tip, after you have deleted them from the library (by selecting them and pressing the delete key), you should go to menu:
iPhoto -> Empty iPhoto Trash
Now they are sent to the disk trash, you can leave them there for a few day, or until you need to empty your trash, and recover disk space.