From iPhoto? Simply start by clicking on the first photo you want to delete. If you want to delete a bunch of them in a row, hold down the Shift key, and then click on the last one in the group you want to delete. In the bottom right-hand corner of the last photo, you will see a little - click on that, and you will see a Trash option - click on that. It will ask if you want to delete the selected photos. Say yes, and they will be deleted.