Inside the photobooth, make Strong Bad walk off the far right side. Do it again, on the third try you should be upside down walking on the bottom of the photobooth. Walk over to the far right, even with the door. Make sure you're at far back as you can be. You will here a confirmation sound, and Strong Bad will automatically be wearing the bonus shirt. It will not return to the rack when you take it off, you will need to do this every time.