Make an android background service that monitors your phone's sensors and detects when the user lifts the phone (ie you want to unlock).
The app must also have an entry in the Settings menu. This allows the user to take and save an image. You can only store 1 image at a time. So if you have taken a picture before, it will be overwritten.
If an image is already stored, the phone automatically opens and opens the stored image at the above event. Otherwise, it will remain closed.