Here is an answer from their FAQ:
"Generally speaking, photos taken with a cell phone’s camera are not high enough in quality to enlarge, but this is not always the case. If your phone prompts you to choose the image size when you save or send a photo, choose the largest option available (usually called “actual size” or “original size”). If you did this and we still emailed you concerning image quality, we recommend choosing a different image taken with a regular camera. Even phones that take photos at a large file size cannot compare in quality to a regular camera."
Click here for more information regarding selecting an image.
May 17, 2013