From the Model Mayhem FAQ:
Print: The image on my prints is poor quality.
You are responsible for the image quality. ModelMayhem isn’t responsible if you use images that are low quality and don’t reproduce well. Our builder will notify you if the resolution drops too much but we still recommend that you use the highest quality images possible. If there was a problem with the reproduction of your prints please refer to the FAQ “My order is damaged. How do I get a replacement?”