Photo Viewer For Large Image can let you view the details of the photo in original size. There are a small viewer which shows you the resized photo and an indicator which shows you the portion of the photo you are viewing.
While you are viewing the photo, you can easily to scroll to the desired location and the indicator will also move to the corresponding position. In this way, you know which portion of the photo you are viewing. In addition, you can move the indicator to the desired location quickly in the smaller viewer and the view of the photo will also be moved to the corresponding location.
If you find some interested part of the photo, you can crop the portion of the photo and save it to the Photo Library.
If you have any questions or suggestions, please let me know.