I went to a great deal of trouble to make it easy for you to customize your imagery, but along the way I found it impossible to make it idiot-proof. Don't add any folders anywhere inside the Graphics folder. Don't delete any folders anywhere inside the Graphics folder. Don't move any folders anywhere inside the Graphics folder. Now, here's how you can customize your images.
Stages are easiest to customize.
Create an image of your desired stage. The image MUST be exactly 640 pixels wide by 480 pixels high. Save it as a PICT file. Give it exactly the same name that you gave it in the Erasmatron. Then put it inside the Stages folder, which is inside the Graphics folder.