The true longest possible line of sight based on normal refraction was researched by Xavier Eguskitza of Bilbao, and he found the line of sight between Cerro Paramillo (Colombia, N 7°06’12” W 75°58’09”) and the Sierra Nevada de Santa Marta, 483km (300 miles) to the north east. The virtual view is shown here, but it is very unlikely that it has been seen, let alone photographed, in practice. It is possible that longer views exist from south west of Paramillo, but despite extensive searching I have found no longer lines of sight anywhere else in the world. Anyone who thinks there is a longer one, please contact me.
Update 23 April: I still have not found any longer lines of sight anywhere other than across the Colombian plain to/from the Santa Marta, but longer lines of sight than the above are theoretically possible. From the west of Paramillo, there is a 491km view from Altos de Tres Morros, and from the south, there is a 500km view from Paramo Santa Ines (W 75°41’33” N 6°46’22”). In perfect visibility, Pico Ojeda (503km) would be visible through binoculars; and with a telescope part of the ridge further east could be visible at 506km. But I doubt if anyone will ever see this for real.