The regular expression:

(?i-msx:\.(?:gif|jpg)$)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    gif                      'gif'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    jpg                      'jpg'
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------