Look for recovery tools on
www.download.com. The ones which are not free may require you to buy before you can recover the images (but you can see that they can be recovered), but one good one that I've used, though a bit slow, is PC Inspector.
Don't worry, all is not lost, if you have not written anything else to the card. Even if you have, you may still be able to recover the ones sitting on areas which are not over-written.