A nitpicker is a person who looks for tiny faults and will not let them go. The picture is the same as nit-picking: someone going through hair for nits, those tiny louse eggs, and then using that picture for a person who picks at every small mistake. We use it about people, not about things. It is informal, and it is almost always unkind. You might say it with a laugh about a friend who checks every comma, or with real annoyance about a boss. Though actually, every team needs someone who catches the small mistakes before a letter goes out — the sting is in the idea that the person cares about the wrong size of thing. You may also see nit-picker with a hyphen; same person.