The Belfast Newsletter has many funeral notices so it’s worth checking them. John’s funeral is listed in the edition of 17th May 1912. It says he died suddenly at Coolattin, Balmoral and “funeral private” so unfortunately doesn’t give the information you want. However you might want to search for Sarah & David to see if their deaths give more information. Tradition is to bury 3 days after the death so try the editions in the 2 days immediately following the death. The Newsletter is on Ancestry.
As for finding photos of them, unless they were famous for some reason and were in the papers, I am not sure how you would find any. Perhaps living descendants might have some. Have you searched for them?
Probate abstracts:
Gilliland David of Galwally Park Knockbreda county Down agent died 9 June 1925 at Chester Administration Belfast 9 July to Harriet Isabel Gilliland widow. Effects £404 15s.
Gilliland, Harriet Isabel of 1 Galwally Park Belfast widow died 27 June 1960 Probate Belfast 1 December to Dorothy Gilliland spinster and Gweneth Leonore Stevenson married woman. Effects £6229 19s. 4d.
It might be worth reading the will to see who these folk are (the will should be in PRONI).