According to Apple, the photos get backed up in iTunes when you do a sync/backup, so connect your phone to iTunes and restore from the latest backup.
BUT It's not a good idea to restore the whole iTunes backup file, because it can wipe all of your iPhone data including the messages, videos, calendars etc.
Alternative, iPhone Data Recovery will be a better choice. It can preview and selectively recover the deleted photos on iPhone what you want and not influence other data. Refer the guide here:

http://www.android-recovery.net/recover-iphone-photos.html