For taking passport photos in the United States, what specifications should I look for when choosing a camera?
For example, some say phone cameras can't be used for taking passport photos, while some said they can. Can the camera in a Palm Pixi Plus? What phones' cameras can?
My worry is that the focal length, sensor size, etc of a phone camera may not qualify for taking passport photos.
Also is a phone camera more prone to distortions (such as barrel distortion)?
The phone you mention MAY be OK.
Trying one is the only sure way to know.
User reviews should give a guide.
You would want good lighting, at least. There are many much better ways to get a quality photo than to use a low-cost-phone camera. The phone's camera is only part of what you get for the purchase price, meaning that the camera itself is a very low cost item.
That Palm Pixi Plus ad says "... Camera: 2 MP, 1600 x 1200 pixels, LED flash ..." I see an ad there for an AT & T Palm Pixi Plus and which this appears to be the same as. It may or may not be. And a Verizon one here
A careful competent person could use a good quality phone camera of that resolution to take passport quality photos.
HOWEVER it is possible for phone cameras of that resolution to produce very poor quality results due to any of a number of reasons. (Genuine resolution may be lower and scaled up, lens may be poor, image processing may be poor, ...).
A phone that is not made by a known quality manufacturer MAY be of good quality but it is less likely to be than one from well known suppliers. I do not know who makes the PPP phone but the fact that it is supplied for use by a number of US phone companies suggests it may be at least reasonable as a phone - but the $40 ebay price does not suggest it is "top class".
Passport photos are in the "lowest common denominator" portrait category right beside your driver's license. A vending machine photo works (and has been used for decades). it's supposed to look like you after a 12-hour flight in rough weather plus a lengthy gate delay at each end, not after a session with a stylist. I don't even bother combing my hair.
The 600x600 pixel requirement means that some phone front cameras aren't good enough. The screen-facing cameras are intended for video calls, not photography. The back camera (or nearly any webcam over $25) will work fine, as will most built-in cameras on many computers but notably NOT the 2015 MacBook (the extra-thin one).
Tim, for a passport size photo, use any camera that gives your 5MP. That will give you a good quality PP photo. But then make sure you follow the other guidlines as stated in the website for other PP requirements. Here is an excellent link - tomsguide on how pixels translate to prints.
Phone cameras can be used for taking passport photos,especially the latest smartphones, such as the iPhone X Max and iPhone XS, Samsung Galaxy Note 9,and Huawei Mate 20,they have terrific cameras that can shoot high-quality images.
However,I suggest against using the front camera, it claims that the Department of State will likely reject photos taken with the so-called selfie camera because of its low-quality. Hence it would be better to have a friend take your photo using the rear camera.