Are you sure you made the Blank image, and named it blank? If I can remember some syntax (please correct me scripters if I am wrong) but the line that is causing the error:
CODE
@result = Bitmap.new('Graphics/System/Blank')
is the one which fetches the blank image from your Graphics System folder.
This is most likely your problem, so go back through the tutorial, and double check you have created the blank image.