A screenshot is a file suitable for the web; it always has a low resolution and is not suitable for printing. On your screen, the image may look good enough, but when you zoom in (which is what happens when your photo is printed on an IXXI), you can see all the small imperfections in the photo very clearly.
When the quality of a photo is not good enough, our system always gives a warning. We then recommend using a different photo, as we cannot guarantee the quality of the image.