This is the only detailed image we have of Triton, by far Neptune's largest moon. It was captured in 1989 by the Voyager 2 spacecraft during its flyby of the system. I got some amazing facts about Triton for you in this thread

Triton is literally one of the coolest places in the solar system: it has average surface temperatures below those of even Pluto (about -235°C). It's covered in icy nitrogen that reflects 70% of the sunlight that hits it. In fact, the blue-green band you see is nitrogen frost!

Triton is the only large moon in the solar system to orbit in the opposite direction that its host planet rotates (retrograde: see image). Its orbit is also highly inclined to Neptune's spin! For these reasons, it's likely Triton is a Pluto-like dwarf planet that was captured

Amazingly Triton even has a thin, mostly nitrogen, atmosphere (check out the image of it below)! The atmosphere is likely produced by cryo-volcanic activity, with geysers erupting nitrogen gas and a hint of methane

A lot of what we know about Uranus and Neptune is from a quick fly-by of each planet: many planetary scientists (at @IcyGiants, including me) want to go back with an orbiter to reveal much more. Images NASA/JPL/USGS+some processing by me & orbit diagram by @PlutoPittbull

Triton is "by far Neptune's largest moon" as it holds 99.5% of all the mass orbiting Neptune, which has 14 known moons. This imbalance could be because Triton collected and/or tossed out existing moons during its capture. Also, here's Triton to scale by NASA/JPL/USGS/G.H.Revera

You can read more about Triton here https://solarsystem.nasa.gov/moons/neptune-moons/triton/in-depth/ and here https://www.nasa.gov/mission_pages/voyager/multimedia/pia00317.html. The wikipedia article is good: https://en.wikipedia.org/wiki/Triton_(moon), many citations point to NASA or science journal articles