I think it is likely printed on paper because it is a screenshot that someone has saved. Could be a screenshot taken while William was speaking with his foster father, while he was away on business (for example). Or it could be a screenshot found online or on someone else's computer.
As another poster linked before, many people do not understand that posting FB pics of children in cute photos is very appealing to many pedos, and they copy these things for their own gratification and trading purposes. Not suggesting this was a posted FB pic, as I really cannot know that.
And I cannot tell why the police allowed this pic in the MSM publication. Maybe sending a message to the perpetrator, but I am not sure what kind of message it is. We found this on your computer, Mr Perpetrator? Or we found this online, we are not far behind you, you think you can hide on the Dark Web/illicit sites but we are here to tell you that you can't? Or this is the little boy that you took, dressed almost exactly as you took him, and we will get you for this?