Portable Network Graphics (PNG) files are files in an image format that uses a lossless form of data compression, and is the favored picture format for the Apple iPhone. When you take a screen capture on the iPhone it is saved automatically to the Camera Roll as a PNG file, and you can view it directly on the iPhone's screen without the need to install any third-party iPhone applications. For you to load a PNG file, it must first have been saved to the iPhone's Camera Roll.
Saving a PNG File to the Camera Roll
Press and hold a PNG picture file displayed within an email or on a webpage. A dialog box will appear, requesting further instructions on what you'd like to do with the picture.
Tap the "Save Image" button. The PNG file will be copied to the iPhone's Camera Roll.
Tap the "Save All Images" button if the email or webpage contains several PNG files. All PNG files at that location will be copied to the iPhone's Camera Roll at once, so you don't have to save each one individually.
Viewing PNG Files on the iPhone
Launch the Photos application on the iPhone by tapping its icon on the home screen.
Tap the "Camera Roll" folder in the "Albums" section to display thumbnails of all images and videos stored in the iPhone's Camera Roll.
Tap the thumbnail for the PNG file(s) you just saved to display the image in full screen.
Swipe left and right to move between images stored in the Camera Roll.
Tap anywhere on the screen to reveal the Photo application's controls and press the "Camera Roll" button in the upper left corner to return to the album overview.
The Camera Roll is also accessible through the iPhone's Camera application by tapping the image button in the bottom left corner of the application's screen. You can import any PNG images saved to the Camera Roll to a computer by connecting the iPhone using its USB cable.