The iPhoto app is pretty special in that it has its own Trash. When you delete photos from iPhoto they don’t go to the main Trash, but to a separate Trash (found in the iPhoto sidebar).
You can click on this to view all the images you have deleted.
You can also Control + Click the Trash icon in iPhoto to get the Empty Trash command.
This enables you to reclaim hard drive space by deleting the Trash.