Your photo must be saved in the JPEG file format. To see if your picture is a JPEG file in Windows, right click on the file and choose "properties". In Apple you can click your file and go to File > Get Info. It should be JPEG or jpg (or similiar).
Larger photos work best learn more
Larger pictures will work best. If the photo that you upload is too small (less than 600x900) the print quality may be reduced. Make sure that you upload the largest version of the photo that you have.