In this post, René Schulte demonstrates how to load an image from the picture library with the PhotoChooserTask and how to save a picture to the library.
I've built a Windows Phone 7 app that uses the WriteableBitmapEx library and lets the user draw on the WriteableBitmap surface. The app supports multi touch for drawing and the radius of the pen can be changed with the Slider control. An image from the phone's picture library can be chosen with the folder icon button and the floppy disk button saves the current image to the picture library. When the third button (download icon) is clicked, all pictures from the phone's library are loaded without further user interaction. The trash button clears the draw surface.