Look at the file name "United_HK_IDEN_T3902-1"

T = Telephone

3902 is a Metro Area Prefix for Hong Kong (+852 3902 XXXX)

This could be a telephone clue:

either

+852 3902 XXXX

or

+852 3902 1XXX

United NO LONGER FLYS THE 747. Therefore this picture is at least 8 months old, if not longer (assuming that 747 was a commercial United flight).

United’s final 747 flight was on Nov. 7, 2017. It departed San Francisco landed in Honolulu.

Mar. 26, 2017 was last United Boeing 747 flight on the Hong Kong-San Francisco Route

United currently has 4 non-stop international routes from Hong Kong (HKG):

Chicago, IL (ORD - O'Hare)

Guam (GUM)

New York/Newark, NJ (EWR - Liberty)

San Francisco, CA (SFO)