iPhoto sports a special Trash album that holds all your deleted photos, just like the Finder's Trash. And like the Finder's Trash, you can pull mistakenly deleted photos out.
Ways to recover photos:
Select one or more photos in the Trash album and choose Restore to Photo Library () from the Photos menu (Figure 2.20).
Figure 2.20. To remove photos from the Trash, select them and choose Restore to Photo Library from the Photos menu.
Select one or more photos in the Trash album, -click the selection, and choose Restore to Photo Library from the contextual menu that appears.
Drag one or more photos from the Trash album into the Library album or to the bottom of the Source pane.
You can't edit photos stored in the Trash, nor can you create a book, calendar, or card using the Trash album. Well, duh!
You can't drag a photo from the Trash album into another album without first restoring it to the Library album.
iPhoto doesn't move the actual image files when you put them in the Trash; it merely tracks which ones are in the Trash album. Only when you empty the Trash are the actual files deleted.
Use Your Trash!
I strongly recommend that you make full use of iPhoto's Trash and empty it only occasionally. The whole point of having a Trash is to save you from mistakes, and you never know if you'll realize a mistake right away. Instead, let photos sit in the Trash for a while before deleting them for good. Or wait until you feel like you need the disk space they take up before deleting them.