The thing is, of course a rule is needed regarding the pictures, it's mainly a reference so people can guide and not upload pictures the way each person want, that would result on different sizes and parameters, not good for a selection process or any other kind of process. Things must be organized, some people complain they have exaggerated rules, well it's quite simple actually.
As long you respect the picture size, color, focus, head uncovered and some other small stuff, its the same way you take photos for your passport, it's a standard photo card format but bigger.
The DV photo will not be printed on some official document, it's only for the lottery, if they don't like the photo, they'll ask you another AT the interview after selected.