Added a few images to the example story

Images were added to the example story in order to prepare for the tests
related to image support.
Tested with new story file and corrected bugs

Tested twee2sam with the recently edited story file, and corrected any
bugs that happened to be found.
Implemented image support
Modified the cleanup to delete the PNGs
Corrected empty text bug

The program was showing screens with empty text. This was corrected.
Updated the README to tell about the image display