StillShot is a popular app for iOS photography and video capture, it allows users to create a full resolution photo from video files by loading the video and analyzing the frames one by one, letting the user to choose the best frame. The app can extract frames from any video, with no special requirements. StillShot holds the original resolution of the video, e.g., on iPhone 4s the resolution is 1080p (1920*1080px).
Take a look at the video demonstration below. Usually StillShot retails for $0.99/£0.69 but today you can download it for free, click here.