Stack is a very simple image collage tool. Simply add an image from your photos and use multi-touch gestures to position the image. Touch the image you would like to bring to the top of the stack. You can also change the background colors and draw simple doodles over the photos.
Basic controls:
+ adds an image to the stack
- removes the top image from the stack
Great when you need to doodle on something or just want to send a single image.