Technically, the site can add hidden biometric data to the photo that allows multiple people to travel on the same passport.
They also give you incentive to choose the passport you’re going to use it for, pretending the picker is about the specific regulations of your country’s passport. So they even know what passport to fake that most likely belongs to the person uploading their picture.
You will then likely use the photos you downloaded from their website to get official papers and that’s the jackpot for them.
That’s the main reason you should not use a service like this to save 25 bucks. You might be aiding organised crime and/or foreign governments in obtaining travel documents which allow them to hide their true identity.
In Mali, for example, many people want to flee to Europe. They buy a passport with a morphed photo that belongs to someone around the same age, having the same skin colour and citizenship in a European country.
They also know you must be close to obtaining new documents for whatever reasons, as you specifically are looking for their service, and they can retain a copy of the morphed image to sell a passport with your photo to someone around the same age and skin color in a third-world country for a few thousand dollars, or to hide a Mossad hit team from entering the country to murder one of your fellow citizens.