DD1 needed one last year, and we just went to Kinko's. They're really picky about the pictures, so I didn't want to risk doing one myself and not having it meet the standards.
We got ours done at a photo place that does passport photos. Like pp said, I didn't want to screw up the dimensions, etc. The major requirement, though, is that the baby's eyes are open. It was tough for us b/c we got DD2's photos taken when she was 4 days old. We live overseas, so we wanted to get the passport asap.
We got passports at 6 weeks old. Like pp said the important thing is eyes open. We took her to the passport office because CVS told us that they are not allowed to take the infant pics and only the passport office can do it. The passport offfice is very picky about the pics and taking the pic yourself could delay your passport. I would just take LO to the passport office and have them do it.
Re: if your LO already has a passport...
We got passports at 6 weeks old. Like pp said the important thing is eyes open. We took her to the passport office because CVS told us that they are not allowed to take the infant pics and only the passport office can do it. The passport offfice is very picky about the pics and taking the pic yourself could delay your passport. I would just take LO to the passport office and have them do it.