Time machine backups and deleted files
By default, Time Machine keeps the following backups:


Hourly for the current day
Daily for the past month
Weekly for all the other months


If there's space constraints then Time Machine will start to delete the backups, but otherwise it keeps them for as long as possible.

To get to the individual photos you will have to have iPhoto as the active window when you activate Time Machine from the menu bar. This lets you go back and see all of the photos. If you did it from the Finder it's more complex because iPhoto bundles its whole library into one big package.