Passport Photo

A passport photo is a requirement for most passport/visa documents. Different countries have very different passport photo requirements. The list of all passport photo requirements for 500+ documents can be found here. Most of the passport photos are of the size 2x2 Like the US passport photo or 35x45mm like the European visa photos. Passport photos can be printed at home or from professional studios. Newer websites like https://makepassportphoto.com make it easy to get passport photos online without the hassle. 
Passport Photo requirements
US : The us passport photo requirements are listed here.