Just been through this twice and thought I would share! DS needed passport pictures for his UK and US passports and I've just had them done while he was 6-7 weeks old.
His UK passport photo was taken with him lying on the floor on a white mat. Unfortunately his head kept turning to the side, but I was able to get him looking straight on using a muslin bunched up under his neck. He's basically mid-scream in the photo, but the guy said they aren't too fussy with baby pictures here.
US has different requirements and are a lot stricter; according to the photographer they want the baby's eyes open and mouth closed. Given our first experience, I wasn't sure how I'd be able to do this---then had a flash of inspiration. Every time I've given DS Infacol, he opens his eyes wide in surprise and purses his lips! So I just gave him a little bit, got my hands out of the way quickly, and the guy snapped the photo on the first try! It looks perfect!