As per the developers, ‘Big Photo allows you to view pictures and images in full resolution with unlimited zoom in/out and highest quality. An essential tool for reviewing fine details of panorama pictures of iOS 6 devices or 8MP pictures of iPhone 4S/5. Precisely crop with exact coordinates and pixel sizes, with or without locking custom aspect ratio. Accurately enlarge (like digital zoom) or downsize pictures with exact pixel sizes with or without locking aspect ratio. Automatically save your cropped, enlarged or downsized pictures back to your Camera Roll’.
This app usually retails for $1.99/£1.49 but it is currently free. Click here to download.