Below you will see a high resolution photo taken with a 5.1 megapixel camera. (850k file size)
To the right is the same photo taken with a camera phone. (39k file size).

They both look relatively good from a distance. However the photo at the bottom shows the difference
when they are both made larger for a life size standup.
Her face is still very clear on the left, but becomes very grainy on the right side.

This is why we ask for a high resolution digital photo or a negative when ever possible.