I shared Video to Photo Grabber app days ago. This iOS app lets you extract photos from video clips captured with your iPhone/iPad camera quite easily. The videos in your iPhone/iPad Camera Roll will be loaded immediately after you launch the app. Then you can select any of them and the app will parse it frame by frame. Simply tap the frame photo you need to get it captured. Then you can choose to save the snapshot to the camera roll, or enhance it with the built-in photo editor app. There are options to email the photo, open the photo in other apps, send the photo via iTunes File Sharing, etc. Learn more about how to convert video to photo on iPhone using this app.
I just made a demo for this app on my iPhone 4 and uploaded it to YouTube. You may take a look.
Video to Photo Grabber is available on the iTunes App Store at https://itunes.apple.com/us/app/video-to-photo-grabber/id639485093