You can't for two reasons: First, the iPhone doesn't have a hard drive. Second, there is no Trash folder, so once something is deleted it is gone forever. If you were foresighted enough to sync your phone regularly the photos should still be in the app you used for managing photos from digital cameras on your computer, and they are also in the iPhone backup on your computer. If you enabled Photo Stream they are also in iCloud. But if you didn't take advantage of any of these backup options you are out of luck.