I REGULARLY receive photos from cameras of someones computer screen when asked to supply a "screen shot". It was kinda funny the first few times...now its just sad.
"Cntl-PrintScr" is not an intuitive command to most people. Pressing the shutter button on a digital camera is.
Maybe this is a larger UI point than the author intended -- I'm not sure that the PC, Mac, or iPhone commands are the best way to solve the problem.