What is the purpose of the Photo URL in the import spreadsheet and what is the source of this information?

The Photo URL column of the Contacts tab in the import spreadsheet enables you to add photos to your contacts inside of Headcount. 

The source of these links should be from any publicly accessible photo URL. 

For example, someone's LinkedIn profile photo.

Alternatively, Headcount can host private and secure photos, which is enabled through a separate import process.