tkDerm handles two types of user accounts: the operating system user account and the PostgreSQL user account. Although they are conceptually independent of each other, tkDerm keeps them maintain a correspondence with each other by assigning the same name as the operating system account to the PostgreSQL user account.
You should have an operating system user account before installing tkDerm. When you initialize tkDerm, the tkDerm Setup Wizard will create a PostgreSQL user account whose name is the same as your operating system account and whose password is the one for which you will be prompted during initialization. The user account used when you initialize tkDerm will be treated as the tkDerm superuser, who can do all the functions offered by tkDerm including the ones elicited from the Admin and Package menus, which are not permitted to perform by the other user accounts.
When the tkDerm superuser registers new doctor data, tkDerm will create a PostgreSQL user account, if it has not existed yet, whose name is the same as the operating system user account that should be typed in the Account entry in the Add Doctor data dialog and whose password is the one to be given at registration. The user account so created doesn't have the privileges of the superuser and cannot execute the menu items from the Admin and Package menus and some actions that will result in a change of the photos such as rotating or deleting them.
When tkDerm starts up, it connects to the PostgreSQL server with the account whose name is the same as the operating system account and with the password given at the startup.