Where do people keep their images and look at them most? You guessed it! Facebook. A lot of people see images on Facebook and want the printed as posters. This can be a mistake. Even if the image was taken on a great camera Facebook automatically reduces the resolution to reduce the size so those images can be uploaded faster, browsed faster and take take up much less space on their servers.
Facebook does have the option to upload high resolution images now but there can still be some resolution reduction taking place. The best thing to do is ask the person who uploaded the image to email you the original. BE SURE THEY ATTACH THE IMAGE to the email and do not copy and paste it. This way you will get the best quality poster possible.