What version of iPhoto? That feature was removed in iPhoto '09 I believe - it is not available in iPhoto '11
If you know their file name (typically the camera assigned name) you can use TM to go inside the iPhoto library to the masters folder and restore the original photo to the desktop and then import that to iPhoto
Here's a possible method to recover a single photo from an iPhoto Library in Time Machine:
1 - download and launch the shareware Find Any File application. It can search areas that Spotligh can't.
2 - select your Time Machine drive as the drive to search.
3 - enter the file name of the photo to search for and hit Find:
It will be a very slow search due to the nature of the Time Machine structure.
4 - When the file is located you can drag it from the search results window to your Desktop.