how do I edit a photo that 's already in Pictures on my iPad2?
There is a button at the bottom of PST's initial screen which allows you to import an image and start a project. You can also start a project and load an image with one of the buttons on the top left. BTW I use the Android version, not iPad, so I can't say for sure that the buttons are exactly where I said.
North America
Europe, Middle East and Africa
Asia Pacific