So why do they form behind engines instead of over the full width of the wing?
The condensation shown in those other photos is purely localised over the wing and doesn't leave a trail, and it only occurs in high-lift situations when the wing is in a very high angle of attack. You can get that from airliners on take off and landing, but you won't get it when flying straight and level. Even if it was happening at 35,000ft, because you're so far away (and looking from below - the condensation occurs above the wing) you won't see it as there's no "trail".
You
do get trails from vortices which only occur at the tip of wings or at other interruptions of the wing plane, however they're less likely to occur at higher altitudes because
A) Air is thinner, so there's necessarily a lower concentration of water vapour to condense - in contrast the water vapour that condenses to form your classic contrails comes from the engines
B) The air is very cold, so there's less likelihood of the vortex core causing a sufficient shift in air temperature to cause condensation - in contrast your classic contrail is formed because the water vapour in the engine exhaust is warm, and cools as it hits the cold air
C) Vortices are a by product of wings generating lift and are greater at high angles of attack, and particularly when flaps or spoilers are deployed - they're more likely to produce contrails at lower altitude and when taking off and landing
D) Vortices are unavoidable but they're also a
bad thing because they reduce the amount of lift a wing can generate. Airliners have long thin wings in order to reduce the amount of vortices, at the sacrifice of maneuverability (fighter aircraft, with their short stubby wings, sacrifice lift efficiency and generate much greater vortices for their size). Many airliners go the further step of reducing vortices even more by having vertical winglets at the end of the wings. The larger the vortex, the faster it spins and therefore the greater the pressure drop in the core, thus the more likely you'll get a contrail.
Because the contrails from prop-driven aircraft and helicopters are also caused by airfoils (the propeller blades) the same general principles apply as for wings listed above.
Hope that explains it.
