You select collections of users in the following ways:
All users: gam all users ...
All users directly in an OU: gam ou <PathOfOU> ...
All users in an OU and its sub OUs: gam ou_and_children <PathOfOU> ...
All users in a group: gam group <NameOgGroup> ...
A list of users: gam users <NameOfUser>,<NameOfUser> ...
A single user: gam user <NameOfUser> ...
A list of users based on a query: gam query <Query> ...
A list of users from a flat file: gam file <Filename> ...
The flat file can be a CSV file, the users are taken from the last column, there should be no header
In your case: gam ou /Staff update photo c:\photos\#user#.jpg