We took our lad to Japan when he was 6 months and faced the same issue; the least stressful way for us was to use our own camera and lie him down on the bed with white sheets behind him. Could then snap away, pick the best and alter the dimensions, etc on a computer.
It's definitely worth having the picture taken by someone who knows what the passport office will/won't accept. Although it's a while back now, we had huge frustration getting the right picture of our 2 week old for her passport. After much twisting and contortions in a photo booth, we were sent back twice by the passport office and had to get the picture re-done and signed all ove again. CAM pharmacy opposite Kennington Road Police station eventually came up with the right one.