USERSIG -- show user signature with profile picture icon

• Syntax: %USERSIG{WikiName}% or %USERSIG{WikiName - YYYY-MM-DD}%
• The parameter can be either just the WikiName of a user, or by convention the WikiName followed by a dash and the date
• Example:
  -- %USERSIG{TWikiGuest - 2023-10-01}%
• Expands to:
  -- TWiki Guest - 2023-10-01
• Category: EditingAndContentUpdateVariables, FormattingAndRenderingVariables, UsersAndAuthenticationVariables
• Related: BUBBLESIG, USERREPORT, UserList, UserReports