When you delete photos from your memory card, they aren't actually deleted. They are still there on the card, but the information on how to retrieve them from the card is erased so that you no longer see them or can get to them. Because of this, if you accidentally delete photos from your memory card, you can still get them back.

CardRaider (http://www.ecamm.com/mac/cardraider/) is a Mac program you can use to recover lost photos. The key to recovering deleted photos is to *stop* using the memory card as soon as possible. When you use the card to save new photos, the camera may start overwriting old photos that have been deleted. Once a deleted photo is overwritten, it can not be recovered, no matter what software you have.